Sample records for circular quantum dots

We adapt the transfer matrix (T-matrix) method originally designed for one-dimensional quantum mechanical problems to solve the circularly symmetric two-dimensional problem of graphene quantumdots. Similar to one-dimensional problems, we show that the generalized T-matrix contains rich information about the physical properties of these quantumdots. In particular, it is shown that the spectral equations for bound states as well as quasi-bound states of a circular graphene quantumdot and related quantities such as the local density of states and the scattering coefficients are all expressed exactly in terms of the T-matrix for the radial confinement potential. As an example, we use the developed formalism to analyse physical aspects of a graphene quantumdot induced by a trapezoidal radial potential. Among the obtained results, it is in particular suggested that the thermal fluctuations and electrostatic disorders may appear as an obstacle to controlling the valley polarization of Dirac electrons.

This paper shows that negative circular polarization, a spin flip of polarized carriers resulting in emission of opposite helicity, can be observed in undoped, n-doped, and p-doped InAs/GaAs quantumdots. These results contradict the usual interpretation of the effect. We show using power dependent and time resolved spectroscopy that the generation of negative circular polarization correlates with excited state emission. Furthermore, a longer spin lifetime of negatively polarized excitons is observed where emission is largely ground state in character.

We perform coupled-cluster and diffusion Monte Carlo calculations of the energies of circularquantumdots up to 20 electrons. The coupled-cluster calculations include triples corrections and a renormalized Coulomb interaction defined for a given number of low-lying oscillator shells. Using such a renormalized Coulomb interaction brings the coupled-cluster calculations with triples correlations in excellent agreement with the diffusion Monte Carlo calculations. This opens up perspectives for doing ab initio calculations for much larger systems of electrons.

The aim of this work was to design and control, using genetic algorithm (GA) for parameter optimization, one-charge-qubit quantum logic gates σ x, σ y, and σ z, using two bound states as a qubit space, of circular graphene quantumdots in a homogeneous magnetic field. The method employed for the proposed gate implementation is through the quantum dynamic control of the qubit subspace with an oscillating electric field and an onsite (inside the quantumdot) gate voltage pulse with amplitude and time width modulation which introduce relative phases and transitions between states. Our results show that we can obtain values of fitness or gate fidelity close to 1, avoiding the leakage probability to higher states. The system evolution, for the gate operation, is presented with the dynamics of the probability density, as well as a visualization of the current of the pseudospin, characteristic of a graphene structure. Therefore, we conclude that is possible to use the states of the graphene quantumdot (selecting the dot size and magnetic field) to design and control the qubit subspace, with these two time-dependent interactions, to obtain the optimal parameters for a good gate fidelity using GA.

We show that there exists a non-trivial topological phase in circular two-dimensional quantumdots with an odd number of electrons. The possible non-zero value of this phase is explained by axial symmetry of two-dimensional quantum systems. The particular value of this phase (π ) is fixed by T-invariance and the Pauli exclusion principle and leads to half-integer values of the angular orbital momentum for ground states of such systems. This conclusion agrees with the experimental data for ground-state energies of few-electron circularquantumdots in perpendicular magnetic field (Schmidt et al. in Phys Rev B 51:5570, 1995). Hence, these data may be considered as the first experimental evidence for the existence of topological phase leading to half-integer quantization of the orbital angular momentum in circularquantumdots with an odd number of electrons.

Novel types of electro- and photoactive quantumdot-doped cholesteric materials have been engineered. UV-irradiation or electric field application allows one to control the degree of circular polarization and intensity of fluorescence emission by prepared quantumdot-doped liquid crystal films. PMID:22972420

We propose a new type of chiral metamaterial based on an ensemble of artificial molecules formed by three identical quantumdots in a triangular arrangement. A static magnetic field oriented perpendicular to the plane breaks mirror symmetry, rendering the molecules sensitive to the circular polarization of light. By varying the orientation and magnitude of the magnetic field one can control the polarization and frequency of the emission spectrum. We identify a threshold frequency Ω, above which we find strong birefringence. In addition, Kerr rotation and circular-polarized lasing action can be implemented. We investigate the single-molecule lasing properties for different energy-level arrangements and demonstrate the possibility of circular-polarization conversion. Finally, we analyze the effect of weak stray electric fields or deviations from the equilateral triangular geometry. PMID:25526146

We propose a new type of chiral metamaterial based on an ensemble of artificial molecules formed by three identical quantumdots in a triangular arrangement. A static magnetic field oriented perpendicular to the plane breaks mirror symmetry, rendering the molecules sensitive to the circular polarization of light. By varying the orientation and magnitude of the magnetic field one can control the polarization and frequency of the emission spectrum. We identify a threshold frequency Ω , above which we find strong birefringence. In addition, Kerr rotation and circular-polarized lasing action can be implemented. We investigate the single-molecule lasing properties for different energy-level arrangements and demonstrate the possibility of circular-polarization conversion. Finally, we analyze the effect of weak stray electric fields or deviations from the equilateral triangular geometry.

Coupling nanoscale emitters via optical antennas enables comprehensive control of photon emission in terms of intensity, directivity and polarization. In this work we report highly directional emission of circularly polarized photons from quantumdots coupled to a spiral optical antenna. The structural chirality of the spiral antenna imprints spin state to the emitted photons. Experimental results reveal that a circular polarization extinction ratio of 10 is obtainable. Furthermore, increasing the number of turns of the spiral gives rise to higher antenna gain and directivity, leading to higher field intensity and narrower angular width of emission pattern in the far field. For a five-turn Archimedes' spiral antenna, field intensity increase up to 70-fold simultaneously with antenna directivity of 11.7 dB has been measured in the experiment. The highly directional circularly polarized photon emission from such optically coupled spiral antenna may find important applications in single molecule sensing, quantum optics information processing and integrated photonic circuits as a nanoscale spin photon source. PMID:23038571

We design a circular plasmonic lens for collimation of light emission from nanocrystal quantumdots at room temperature in the near IR spectral range. We implement a two-dimensional k-space imaging technique to obtain the full spectral-angular response of the surface plasmon resonance modes of the bare plasmonic lens. This method is also used to map the full spectral-angular emission from nanocrystal quantumdots positioned at the center of the circular plasmonic lens. A narrow directional emitting beam with a divergence angle of only ∼4.5° full width at half-maximum is achieved with a spectrally broad bandwidth of 30 nm. The spectrally resolved k-space imaging method allows us to get a direct comparison between the spectral-angular response of the resonant surface plasmon modes of the lens and the emission pattern of the quantumdots. This comparison gives a clear and detailed picture of the direct role of these resonant surface waves in directing the emission. The directional emission effect agrees well with calculations based on the coupled mode method. These results are a step toward fabricating an efficient room-temperature single photon source based on nanocrystal quantumdots. PMID:25153365

Confined states of a positronium (Ps) in the spherical and circularquantumdots (QDs) are theoretically investigated in two size quantization regimes: strong and weak. Two-band approximation of Kane’s dispersion law and parabolic dispersion law of charge carriers are considered. It is shown that electron-positron pair instability is a consequence of dimensionality reduction, not of the size quantization. The binding energies for the Ps in circular and spherical QDs are calculated. The Ps formation dependence on the QD radius is studied. PMID:23826867

A new circular dichroism sensor for detecting Ni(2+) and Co(2+) was proposed for the first time using chiral chelating quantumdots. The detection principle was based on changing of circular dichroism signals of the chiral quantumdots when forming a chiral complex with Ni(2+) or Co(2+). L-Cysteine capped cadmium sulfide quantumdots (L-Cyst-CdS QDs) were proposed as a chiral probe. The CD spectrum of L-Cyst-CdS QDs was significantly changed in the presence of Ni(2+) and Co(2+). On the other hand, other studied cations did not alter the original CD spectrum. Moreover, when increasing the concentration of Ni(2+) or Co(2+), the intensity of the CD spectrum linearly increased as a function of concentration and could be useful for the quantitative analysis. The proposed CD sensor showed linear working concentration ranges of 10-60 μM and 4-80 μM with low detection limits of 7.33 μМ and 1.13 μM for the detection of Ni(2+) and Co(2+), respectively. Parameters possibly affected the detection sensitivity such as solution pH and incubation time were studied and optimized. The proposed sensor was applied to detect Ni(2+) and Co(2+) in real water samples, and the results agreed well with the analysis using the standard ICP-OES. PMID:25813022

Experimental investigation of circular dichroism (CD) spectra of complexes based on ZnS:Mn/ZnS and CdSe/ZnS QDs and chlorin e6 (Ce6) molecules in aqua solutions at different pH level, in methanol and in DMSO were carried out. The changes in CD spectra of Ce6 upon its bonding in complex with semiconductor QDs were analyzed. Application of CD spectroscopy allowed to obtain the CD spectrum of luminescent Ce6 dimer for the first time, and to discover a nonluminescent Ce6 aggregate, preliminary identified as a "tetramer", dissymmetry factor of which is 40 times larger than that for its monomer. The analysis of obtained data showed that in complexes with QDs Ce6 can be either in the monomeric form or in the form of non-luminescent tetramer. The interaction of relatively unstable luminescent Ce6 dimerwith QDs leads to its partial monomerization and formation complexes with chlorin e6 in monomeric form.

We demonstrated the lasing action and remarkable reduction in long radiative lifetimes of type-II GaSb/GaAs quantumdots using a circular photonic-crystal nano-cavity with high Purcell factors. The associated enhancement in carrier recombination was surprisingly high and could even surpass type-I counterparts in similar conditions. These phenomena reveal that the type-II sample exhibited extremely low nonradiative recombination so that weak radiative transitions were more dominant than expected. The results indicate that type-II nanostructures may be advantageous for applications which require controllable radiative transitions but low nonradiative depletions.

This review covers the description of the methodologies typically used for the calculation of the electronic structure of self-assembled and colloidal quantumdots. These are illustrated by the results of their application to a selected set of physical effects in quantumdots.

The main goal of our research is to develop new types of technologically important optically active quantumdot (QD) based materials, study their properties and explore their biological applications. For the first time chiral II-VI QDs have been prepared by us using microwave induced heating with the racemic (Rac), D- and L-enantiomeric forms of penicillamine as stabilisers. Circular dichroism (CD) studies of these QDs have shown that D- and L-penicillamine stabilised particles produced mirror image CD spectra, while the particles prepared with a Rac mixture showed only a weak signal. It was also demonstrated that these QDs show very broad emission bands between 400 and 700 nm due to defects or trap states on the surfaces of the nanocrystals. These QDs have demonstrated highly specific chiral recognition of various biological species including aminoacids. The utilisation of chiral stabilisers also allowed the preparation of new water soluble white emitting CdS nano-tetrapods, which demonstrated circular dichroism in the band-edge region of the spectrum. Biological testing of chiral CdS nanotetrapods displayed a chiral bias for an uptake of the D- penicillamine stabilised nano-tetrapods by cancer cells. It is expected that this research will open new horizons in the chemistry of chiral nanomaterials and their application in nanobiotechnology, medicine and optical chemo- and bio-sensing.

We have been investigating the synthesis of quantumdots of CdSe, CuInS2, and CuInSe2 for use in an intermediate bandgap solar cell. We have prepared a variety of quantumdots using the typical organometallic synthesis routes pioneered by Bawendi, et. al., in the early 1990's. However, unlike previous work in this area we have also utilized single-source precursor molecules in the synthesis process. We will present XRD, TEM, SEM and EDS characterization of our initial attempts at fabricating these quantumdots. Investigation of the size distributions of these nanoparticles via laser light scattering and scanning electron microscopy will be presented. Theoretical estimates on appropriate quantumdot composition, size, and inter-dot spacing along with potential scenarios for solar cell fabrication will be discussed.

We have been investigating the synthesis of quantumdots of CdSe, CuInS2, and CuInSe2 for use in an intermediate bandgap solar cell. We have prepared a variety of quantumdots using the typical organometallic synthesis routes pioneered by Bawendi, et. al., in the early 1990's. However, unlike previous work in this area we have also utilized single-source precursor molecules in the synthesis process. We will present XRD, TEM, SEM and EDS characterization of our initial attempts at fabricating these quantumdots. Investigation of the size distributions of these nanoparticles via laser light scattering and scanning electron microscopy will be presented. Theoretical estimates on appropriate quantumdot composition, size, and inter-dot spacing along with potential scenarios for solar cell fabrication will be discussed.

Chiral nanostructures from metals and semiconductors attract wide interest as components for polarization-enabled optoelectronic devices. Similarly to other fields of nanotechnology, graphene-based materials can greatly enrich physical and chemical phenomena associated with optical and electronic properties of chiral nanostructures and facilitate their applications in biology as well as other areas. Here, we report that covalent attachment of l/d-cysteine moieties to the edges of graphene quantumdots (GQDs) leads to their helical buckling due to chiral interactions at the "crowded" edges. Circular dichroism (CD) spectra of the GQDs revealed bands at ca. 210-220 and 250-265 nm that changed their signs for different chirality of the cysteine edge ligands. The high-energy chiroptical peaks at 210-220 nm correspond to the hybridized molecular orbitals involving the chiral center of amino acids and atoms of graphene edges. Diverse experimental and modeling data, including density functional theory calculations of CD spectra with probabilistic distribution of GQD isomers, indicate that the band at 250-265 nm originates from the three-dimensional twisting of the graphene sheet and can be attributed to the chiral excitonic transitions. The positive and negative low-energy CD bands correspond to the left and right helicity of GQDs, respectively. Exposure of liver HepG2 cells to L/D-GQDs reveals their general biocompatibility and a noticeable difference in the toxicity of the stereoisomers. Molecular dynamics simulations demonstrated that d-GQDs have a stronger tendency to accumulate within the cellular membrane than L-GQDs. Emergence of nanoscale chirality in GQDs decorated with biomolecules is expected to be a general stereochemical phenomenon for flexible sheets of nanomaterials. PMID:26743467

We present a lateral quantumdot device which has a tunable number of quantumdots. Depending on easily tunable gate voltages, one, two or three quantumdots are found. They are investigated in transport and charge detection.

Recently, the use of stereospecific chiral stabilising molecules has also opened another avenue of interest in the area of quantumdot (QD) research. The main goal of our research is to develop new types of technologically important quantumdot materials containing chiral defects, study their properties and explore their applications. The utilisation of chiral penicillamine stabilisers allowed the preparation of new water soluble white emitting CdS quantum nanostructures which demonstrated circular dichroism in the band-edge region of the spectrum. It was also demonstrated that all three types of QDs (D-, L-, and Rac penicillamine stabilised) show very broad emission bands between 400 and 700 nm due to defects or trap states on the surfaces of the nanocrystals. In this work the chiral CdS based quantum nanostructures have also been doped by copper metal ions and new chiral penicilamine stabilized CuS nanoparticles have been prepared and investigated. It was found that copper doping had a strong effect at low levels in the synthesis of chiral CdS nanostructures. We expect that this research will open new horizons in the chemistry of chiral nanomaterials and their application in biotechnology, sensing and asymmetric synthesis.

Conspectus Pairs of coupled quantumdots with controlled coupling between the two potential wells serve as an extremely rich system, exhibiting a plethora of optical phenomena that do not exist in each of the isolated constituent dots. Over the past decade, coupled quantum systems have been under extensive study in the context of epitaxially grown quantumdots (QDs), but only a handful of examples have been reported with colloidal QDs. This is mostly due to the difficulties in controllably growing nanoparticles that encapsulate within them two dots separated by an energetic barrier via colloidal synthesis methods. Recent advances in colloidal synthesis methods have enabled the first clear demonstrations of colloidal double quantumdots and allowed for the first exploratory studies into their optical properties. Nevertheless, colloidal double QDs can offer an extended level of structural manipulation that allows not only for a broader range of materials to be used as compared with epitaxially grown counterparts but also for more complex control over the coupling mechanisms and coupling strength between two spatially separated quantumdots. The photophysics of these nanostructures is governed by the balance between two coupling mechanisms. The first is via dipole–dipole interactions between the two constituent components, leading to energy transfer between them. The second is associated with overlap of excited carrier wave functions, leading to charge transfer and multicarrier interactions between the two components. The magnitude of the coupling between the two subcomponents is determined by the detailed potential landscape within the nanocrystals (NCs). One of the hallmarks of double QDs is the observation of dual-color emission from a single nanoparticle, which allows for detailed spectroscopy of their properties down to the single particle level. Furthermore, rational design of the two coupled subsystems enables one to tune the emission statistics from single

Pairs of coupled quantumdots with controlled coupling between the two potential wells serve as an extremely rich system, exhibiting a plethora of optical phenomena that do not exist in each of the isolated constituent dots. Over the past decade, coupled quantum systems have been under extensive study in the context of epitaxially grown quantumdots (QDs), but only a handful of examples have been reported with colloidal QDs. This is mostly due to the difficulties in controllably growing nanoparticles that encapsulate within them two dots separated by an energetic barrier via colloidal synthesis methods. Recent advances in colloidal synthesis methods have enabled the first clear demonstrations of colloidal double quantumdots and allowed for the first exploratory studies into their optical properties. Nevertheless, colloidal double QDs can offer an extended level of structural manipulation that allows not only for a broader range of materials to be used as compared with epitaxially grown counterparts but also for more complex control over the coupling mechanisms and coupling strength between two spatially separated quantumdots. The photophysics of these nanostructures is governed by the balance between two coupling mechanisms. The first is via dipole-dipole interactions between the two constituent components, leading to energy transfer between them. The second is associated with overlap of excited carrier wave functions, leading to charge transfer and multicarrier interactions between the two components. The magnitude of the coupling between the two subcomponents is determined by the detailed potential landscape within the nanocrystals (NCs). One of the hallmarks of double QDs is the observation of dual-color emission from a single nanoparticle, which allows for detailed spectroscopy of their properties down to the single particle level. Furthermore, rational design of the two coupled subsystems enables one to tune the emission statistics from single photon

Quantumdots (QDs) exhibit many characteristics of simpler two-level (or few level) systems, under optical excitation. This makes atomic coherent optical spectroscopy theory and techniques well suited for understanding the behavior of quantumdots. Furthermore, the combination of the solid state nature of quantumdots and their close approximation to atomic systems makes them an attractive platform for quantum information based technologies. In this chapter, we will discuss recent studies using direct detection of light emitted from a quantumdot to investigate coherence properties and confirm entanglement between the emitted photon and an electron spin qubit confined to the QD.

Incorporating nanocrystals into future electronic or optoelectronic devices will require a means of controlling charge-injection processes and an understanding of how the injected charges affect the properties of nanocrystals. We show that the optical properties of colloidal semiconductor nanocrystal quantumdots can be tuned by an electrochemical potential. The injection of electrons into the quantum-confined states of the nanocrystal leads to an electrochromic response, including a strong, size-tunable, midinfrared absorption corresponding to an intraband transition, a bleach of the visible interband exciton transitions, and a quench of the narrow band-edge photoluminescence. PMID:11264530

In this paper, we demonstrate theoretically that the Quantumdots are quite interesting for the electronics industry. Semiconductor quantumdots (QDs) are nanometer-scale crystals, which have unique photo physical, quantum electrical properties, size-dependent optical properties, There small size means that electrons do not have to travel as far as with larger particles, thus electronic devices can operate faster. Cheaper than modern commercial solar cells while making use of a wider variety of photon energies, including "waste heat" from the sun's energy. Quantumdots can be used in tandem cells, which are multi junction photovoltaic cells or in the intermediate band setup. PbSe (lead selenide) is commonly used in quantumdot solar cells.

Often real samples of graphene consist of islands of both monolayer and bilayer graphene. Bound states in such hybrid quantumdots are investigated for (i) a circular single-layer graphene quantumdot surrounded by an infinite bilayer graphene sheet and (ii) a circular bilayer graphene quantumdot surrounded by an infinite single-layer graphene. Using the continuum model and applying zigzag boundary conditions at the single-layer-bilayer graphene interface, we obtain analytical results for the energy levels and the corresponding wave spinors. Their dependence on perpendicular magnetic and electric fields are studied for both types of quantumdots. The energy levels exhibit characteristics of interface states, and we find anticrossings and closing of the energy gap in the presence of a bias potential.

The possibility of building a computer that takes advantage of the most subtle nature of quantum physics has been driving a lot of research in atomic and solid state physics for some time. It is still not clear what physical system or systems can be used for this purpose. One possibility that has been attracting significant attention from researchers is to use the spin state of an electron confined in a semiconductor quantumdot. The electron spin is magnetic in nature, so it naturally is well isolated from electrical fluctuations that can a loss of quantum coherence. It can also be manipulated electrically, by taking advantage of the exchange interaction. In this work we describe several experiments we have done to study the electron spin properties of lateral quantumdots. We have developed lateral quantumdot devices based on the silicon metal-oxide-semiconductor transistor, and studied the physics of electrons confined in these quantumdots. We measured the electron spin excited state lifetime, which was found to be as long as 30 ms at the lowest magnetic fields that we could measure. We fabricated and characterized a silicon double quantumdot. Using this double quantumdot design, we fabricated devices which combined a silicon double quantumdot with a superconducting microwave resonator. The microwave resonator was found to be sensitive to two-dimensional electrons in the transistor channel, which we measured and characterized. We developed a new method for extracting information from random telegraph signals, which are produced when we observe thermal fluctuations of electrons in quantumdots. The new statistical method, based on the hidden Markov model, allows us to detect spin-dependent effects in such fluctuations even though we are not able to directly observe the electron spin. We use this analysis technique on data from two experiments involving gallium arsenide quantumdots and use it to measure spin-dependent tunneling rates. Our results advance the

We review transport experiments on graphene quantumdots and narrow graphene constrictions. In a quantumdot, electrons are confined in all lateral dimensions, offering the possibility for detailed investigation and controlled manipulation of individual quantum systems. The recently isolated two-dimensional carbon allotrope graphene is an interesting host to study quantum phenomena, due to its novel electronic properties and the expected weak interaction of the electron spin with the material. Graphene quantumdots are fabricated by etching mono-layer flakes into small islands (diameter 60-350 nm) with narrow connections to contacts (width 20-75 nm), serving as tunneling barriers for transport spectroscopy. Electron confinement in graphene quantumdots is observed by measuring Coulomb blockade and transport through excited states, a manifestation of quantum confinement. Measurements in a magnetic field perpendicular to the sample plane allowed to identify the regime with only a few charge carriers in the dot (electron-hole transition), and the crossover to the formation of the graphene specific zero-energy Landau level at high fields. After rotation of the sample into parallel magnetic field orientation, Zeeman spin splitting with a g-factor of g ≈ 2 is measured. The filling sequence of subsequent spin states is similar to what was found in GaAs and related to the non-negligible influence of exchange interactions among the electrons.

We investigate the low temperature (300 mK - 10 K) magnetoconductance of open circular bismuth quantumdots (diameter: 500 nm). The structures are fabricated using a combination of electron beam lithography, lift off and plasma etching techniques on bismuth thin films evaporated on heated SiO2 substrates. We observe reproducible magnetoconductance fluctuations (UCFs) up to 5T, qualitatively similar to conductance fluctuations evidenced in open quantumdots patterned in high mobility semiconductor heterostructures. In our samples, UCFs are superposed on a slowly varying negative magnetoconductance background. We also observe a sharp conductance maximum centered in B=0, which is reminescent of the spin-orbit induced anti-localisation phenomenon. The behavior of UCFs and of the conductance maximum is discussed as a function of the temperature, thickness and degree of cristallinity of the cavity.

We demonstrate an InAs quantumdotquantum cascade infrared photodetector operating at room temperature with a peak detection wavelength of 4.3 μm. The detector shows sensitive photoresponse for normal-incidence light, which is attributed to an intraband transition of the quantumdots and the following transfer of excited electrons on a cascade of quantum levels. The InAs quantumdots for the infrared absorption were formed by making use of self-assembled quantumdots in the Stranski-Krastanov growth mode and two-step strain-compensation design based on InAs/GaAs/InGaAs/InAlAs heterostructure, while the following extraction quantum stairs formed by LO-phonon energy are based on a strain-compensated InGaAs/InAlAs chirped superlattice. Johnson noise limited detectivities of 3.64 × 1011 and 4.83 × 106 Jones at zero bias were obtained at 80 K and room temperature, respectively. Due to the low dark current and distinct photoresponse up to room temperature, this device can form high temperature imaging.

We demonstrate an InAs quantumdotquantum cascade infrared photodetector operating at room temperature with a peak detection wavelength of 4.3 μm. The detector shows sensitive photoresponse for normal-incidence light, which is attributed to an intraband transition of the quantumdots and the following transfer of excited electrons on a cascade of quantum levels. The InAs quantumdots for the infrared absorption were formed by making use of self-assembled quantumdots in the Stranski–Krastanov growth mode and two-step strain-compensation design based on InAs/GaAs/InGaAs/InAlAs heterostructure, while the following extraction quantum stairs formed by LO-phonon energy are based on a strain-compensated InGaAs/InAlAs chirped superlattice. Johnson noise limited detectivities of 3.64 × 10{sup 11} and 4.83 × 10{sup 6} Jones at zero bias were obtained at 80 K and room temperature, respectively. Due to the low dark current and distinct photoresponse up to room temperature, this device can form high temperature imaging.

Semiconductor quantumdots have replaced conventional inorganic phosphors in numerous applications. Despite their overall successes as emitters, their impact as laser materials has been severely limited. Eliciting stimulated emission from quantumdots requires excitation by intense short pulses of light typically generated using other lasers. In this Letter, we develop a new class of quantumdots that exhibit gain under conditions of extremely low levels of continuous wave illumination. We observe thresholds as low as 74 mW/cm(2) in lasers made from these materials. Due to their strong optical absorption as well as low lasing threshold, these materials could possibly convert light from diffuse, polychromatic sources into a laser beam. PMID:26978011

Spectroscopy is carried out in almost every field of science, whenever light interacts with matter. Although sophisticated instruments with impressive performance characteristics are available, much effort continues to be invested in the development of miniaturized, cheap and easy-to-use systems. Current microspectrometer designs mostly use interference filters and interferometric optics that limit their photon efficiency, resolution and spectral range. Here we show that many of these limitations can be overcome by replacing interferometric optics with a two-dimensional absorptive filter array composed of colloidal quantumdots. Instead of measuring different bands of a spectrum individually after introducing temporal or spatial separations with gratings or interference-based narrowband filters, a colloidal quantumdot spectrometer measures a light spectrum based on the wavelength multiplexing principle: multiple spectral bands are encoded and detected simultaneously with one filter and one detector, respectively, with the array format allowing the process to be efficiently repeated many times using different filters with different encoding so that sufficient information is obtained to enable computational reconstruction of the target spectrum. We illustrate the performance of such a quantumdot microspectrometer, made from 195 different types of quantumdots with absorption features that cover a spectral range of 300 nanometres, by measuring shifts in spectral peak positions as small as one nanometre. Given this performance, demonstrable avenues for further improvement, the ease with which quantumdots can be processed and integrated, and their numerous finely tuneable bandgaps that cover a broad spectral range, we expect that quantumdot microspectrometers will be useful in applications where minimizing size, weight, cost and complexity of the spectrometer are critical.

Engineering the spectral properties of quantumdots can be achieved by a control of the quantumdots organization on a substrate. Indeed, many applications of quantumdots as LEDs are based on the realization of a 3D architecture of quantumdots. In this contribution, we present a systematic study of the quantumdot organization obtained on different chemically modified substrates. By varying the chemical affinity between the quantumdots and the substrate, the quantumdot organization is strongly modified from the 2D monolayer to the 3D aggregates. Then the photoluminescence of the different obtained samples has been systematically studied and correlated with the quantumdot film organization. We clearly show that the interaction between the substrate and the quantumdot must be stronger than the quantumdot-quantumdot interaction to avoid 3D aggregation and that these organization strongly modified the photoluminescence of the film rather than intrinsic changes of the quantumdot induced by pure surface chemistry. PMID:18792763

Recent advances in the application of semiconductor nanocrystals, or quantumdots, as biochemical sensors are reviewed. Quantumdots have unique optical properties that make them promising alternatives to traditional dyes in many luminescence based bioanalytical techniques. An overview of the more relevant progresses in the application of quantumdots as biochemical probes is addressed. Special focus will be given to configurations where the sensing dots are incorporated in solid membranes and immobilized in optical fibers or planar waveguide platforms.

The coherent generation of light, from masers to lasers, relies upon the specific structure of the individual emitters that lead to gain. Devices operating as lasers in the few-emitter limit provide opportunities for understanding quantum coherent phenomena, from terahertz sources to quantum communication. Here we demonstrate a maser that is driven by single-electron tunneling events. Semiconductor double quantumdots (DQDs) serve as a gain medium and are placed inside a high-quality factor microwave cavity. We verify maser action by comparing the statistics of the emitted microwave field above and below the maser threshold. PMID:25593187

As molecular labels for cells and tissues, fluorescent probes have shaped our understanding of biological structures and processes. However, their capacity for quantitative analysis is limited because photon emission rates from multicolour fluorophores are dissimilar, unstable and often unpredictable, which obscures correlations between measured fluorescence and molecular concentration. Here we introduce a new class of light-emitting quantumdots with tunable and equalized fluorescence brightness across a broad range of colours. The key feature is independent tunability of emission wavelength, extinction coefficient and quantum yield through distinct structural domains in the nanocrystal. Precise tuning eliminates a 100-fold red-to-green brightness mismatch of size-tuned quantumdots at the ensemble and single-particle levels, which substantially improves quantitative imaging accuracy in biological tissue. We anticipate that these materials engineering principles will vastly expand the optical engineering landscape of fluorescent probes, facilitate quantitative multicolour imaging in living tissue and improve colour tuning in light-emitting devices.

As molecular labels for cells and tissues, fluorescent probes have shaped our understanding of biological structures and processes. However, their capacity for quantitative analysis is limited because photon emission rates from multicolour fluorophores are dissimilar, unstable and often unpredictable, which obscures correlations between measured fluorescence and molecular concentration. Here we introduce a new class of light-emitting quantumdots with tunable and equalized fluorescence brightness across a broad range of colours. The key feature is independent tunability of emission wavelength, extinction coefficient and quantum yield through distinct structural domains in the nanocrystal. Precise tuning eliminates a 100-fold red-to-green brightness mismatch of size-tuned quantumdots at the ensemble and single-particle levels, which substantially improves quantitative imaging accuracy in biological tissue. We anticipate that these materials engineering principles will vastly expand the optical engineering landscape of fluorescent probes, facilitate quantitative multicolour imaging in living tissue and improve colour tuning in light-emitting devices. PMID:26437175

Solotronics, optoelectronics based on solitary dopants, is an emerging field of research and technology reaching the ultimate limit of miniaturization. It aims at exploiting quantum properties of individual ions or defects embedded in a semiconductor matrix. It has already been shown that optical control of a magnetic ion spin is feasible using the carriers confined in a quantumdot. However, a serious obstacle was the quenching of the exciton luminescence by magnetic impurities. Here we show, by photoluminescence studies on thus-far-unexplored individual CdTe dots with a single cobalt ion and CdSe dots with a single manganese ion, that even if energetically allowed, nonradiative exciton recombination through single-magnetic-ion intra-ionic transitions is negligible in such zero-dimensional structures. This opens solotronics for a wide range of as yet unconsidered systems. On the basis of results of our single-spin relaxation experiments and on the material trends, we identify optimal magnetic-ion quantumdot systems for implementation of a single-ion-based spin memory. PMID:24463946

It is well known that a parallel quantum computer is more powerful than a classical one. So far, there are some important works about the construction of universal quantum logic gates, the key elements in quantum computation. However, they are focused on operating on one degree of freedom (DOF) of quantum systems. Here, we investigate the possibility of achieving scalable hyper-parallel quantum computation based on two DOFs of photon systems. We construct a deterministic hyper-controlled-not (hyper-CNOT) gate operating on both the spatial-mode and the polarization DOFs of a two-photon system simultaneously, by exploiting the giant optical circular birefringence induced by quantum-dot spins in double-sided optical microcavities as a result of cavity quantum electrodynamics (QED). This hyper-CNOT gate is implemented by manipulating the four qubits in the two DOFs of a two-photon system without auxiliary spatial modes or polarization modes. It reduces the operation time and the resources consumed in quantum information processing, and it is more robust against the photonic dissipation noise, compared with the integration of several cascaded CNOT gates in one DOF.

We review recent theoretical work on thermoelectric energy harvesting in multi-terminal quantum-dot setups. We first discuss several examples of nanoscale heat engines based on Coulomb-coupled conductors. In particular, we focus on quantumdots in the Coulomb-blockade regime, chaotic cavities and resonant tunneling through quantumdots and wells. We then turn toward quantum-dot heat engines that are driven by bosonic degrees of freedom such as phonons, magnons and microwave photons. These systems provide interesting connections to spin caloritronics and circuit quantum electrodynamics. PMID:25549281

Quantumdots are photostable fluorescent semiconductor nanocrystals possessing wide excitation and bright narrow, symmetrical, emission spectra. These characteristics have engendered considerable interest in their application in multiplex immunohistochemistry for biomarker quantification and co-localisation in clinical samples. Robust quantitation allows biomarker validation, and there is growing need for multiplex staining due to limited quantity of clinical samples. Most reported multiplexed quantumdot staining used sequential methods that are laborious and impractical in a high-throughput setting. Problems associated with sequential multiplex staining have been investigated and a method developed using QDs conjugated to biotinylated primary antibodies, enabling simultaneous multiplex staining with three antibodies. CD34, Cytokeratin 18 and cleaved Caspase 3 were triplexed in tonsillar tissue using an 8 h protocol, each localised to separate cellular compartments. This demonstrates utility of the method for biomarker measurement enabling rapid measurement of multiple co-localised biomarkers on single paraffin tissue sections, of importance for clinical trial studies.

The project objective is to create low cost coatable inorganic light emitting diodes, composed of quantumdot emitters and inorganic nanoparticles, which have the potential for efficiencies equivalent to that of LEDs and OLEDs and lifetime, brightness, and environmental stability between that of LEDs and OLEDs. At the end of the project the Recipient shall gain an understanding of the device physics and properties of Quantum-Dot LEDs (QD-LEDs), have reliable and accurate nanocrystal synthesis routines, and have formed green-yellow emitting QD-LEDs with a device efficiency greater than 3 lumens/W, a brightness greater than 400 cd/m2, and a device operational lifetime of more than 1000 hours. Thus the aim of the project is to break the current cost-efficiency paradigm by creating novel low cost inorganic LEDs composed of inorganic nanoparticles.

The project objective is to create low cost coatable inorganic light emitting diodes, composed of quantumdot emitters and inorganic nanoparticles, which have the potential for efficiencies equivalent to that of LEDs and OLEDs and lifetime, brightness, and environmental stability between that of LEDs and OLEDs. At the end of the project the Recipient shall gain an understanding of the device physics and properties of Quantum-Dot LEDs (QD-LEDs), have reliable and accurate nanocrystal synthesis routines, and have formed green-yellow emitting QD-LEDs with a device efficiency greater than 3 lumens/W, a brightness greater than 400 cd/m{sup 2}, and a device operational lifetime of more than 1000 hours. Thus the aim of the project is to break the current cost-efficiency paradigm by creating novel low cost inorganic LEDs composed of inorganic nanoparticles.

The problem is considered of scattering in a system consisting of a quantum waveguide and a quantumdot weakly coupled to the waveguide. It is assumed that the quantum waveguide is described by the Pauli equations, and the Rashba spin-orbit interaction is taken into account. The possibility of tunneling through the quantumdot is proved.

A generalized drift-diffusion model for the calculation of both the quantumdot filling profile and the dark current of quantumdot infrared photodetectors is proposed. The confined electrons inside the quantumdots produce a space-charge potential barrier between the two contacts, which controls the quantumdot filling and limits the dark current in the device. The results of the model reasonably agree with a published experimental work. It is found that increasing either the doping level or the temperature results in an exponential increase of the dark current. The quantumdot filling turns out to be nonuniform, with a dot near the contacts containing more electrons than one in the middle of the device where the dot occupation approximately equals the number of doping atoms per dot, which means that quantumdots away from contacts will be nearly unoccupied if the active region is undoped.

We demonstrate optical control of the polarization eigenstates of a neutral quantumdot exciton without any external fields. By varying the excitation power of a circularly polarized laser in microphotoluminescence experiments on individual InGaAs quantumdots we control the magnitude and direction of an effective internal magnetic field created via optical pumping of nuclear spins. The adjustable nuclear magnetic field allows us to tune the linear and circular polarization degree of the neutral exciton emission. The quantumdot can thus act as a tunable light polarization converter. PMID:19792745

It is proposed to use quantumdots in order to increase the temperatures suitable for observation of the integer quantum Hall effect. A simple estimation using Fock-Darwin spectrum of a quantumdot shows that good part of carriers localized in quantumdots generate the intervals of plateaus robust against elevated temperatures. Numerical calculations employing local trigonometric basis and highly efficient kernel polynomial method adopted for computing the Hall conductivity reveal that quantumdots may enhance peak temperature for the effect by an order of magnitude, possibly above 77 K. Requirements to potentials, quality and arrangement of the quantumdots essential for practical realization of such enhancement are indicated. Comparison of our theoretical results with the quantum Hall measurements in InAs quantumdot systems from two experimental groups is also given.

Semiconductor nanocrystals, or quantumdots (QDs), are candidates for biological sensing, photovoltaics, and catalysis due to their unique photophysical properties. The most studied QDs are composed of heavy metals like cadmium and lead. However, this engenders concerns over heavy metal toxicity. To address this issue, numerous studies have explored the development of nontoxic (or more accurately less toxic) quantumdots. In this Review, we select three major classes of nontoxic quantumdots composed of carbon, silicon and Group I-III-VI elements and discuss the myriad of synthetic strategies and surface modification methods to synthesize quantumdots composed of these material systems. PMID:26548450

The coherent coupling of quantumdots is a sensitive indicator of the energy and phase relaxation processes taking place in the nanostructure components. We formulate a theory of low-temperature, stationary photoluminescence from a quantum-dot molecule composed of two spherical quantumdots whose electronic subsystems are resonantly coupled via the Coulomb interaction. We show that the coupling leads to the hybridization of the first excited states of the quantumdots, manifesting itself as a pair of photoluminescence peaks with intensities and spectral positions strongly dependent on the geometric, material, and relaxation parameters of the quantum-dot molecule. These parameters are explicitly contained in the analytical expression for the photoluminescence differential cross section derived in the paper. The developed theory and expression obtained are essential in interpreting and analyzing spectroscopic data on the secondary emission of coherently coupled quantum systems.

The coupled quantumdots can be designed to possess negative polarizability in low-lying excited states. In an electric field, the coupled dots are polarized, and the dipole moment of the coupled dots is reversed by absorbing one photon. This photoswitch effect is a new photoinduced phenomenon.

Semiconductor quantumdots provide a two-dimensional analogy for real atoms and show promise for the implementation of scalable quantum computers. Here, we investigate the charge configurations in a silicon metal-oxide-semiconductor double quantumdot tunnel coupled to a single reservoir of electrons. By operating the system in the few-electron regime, the stability diagram shows hysteretic tunnelling events that depend on the history of the dots charge occupancy. We present a model which accounts for the observed hysteretic behaviour by extending the established description for transport in double dots coupled to two reservoirs. We demonstrate that this type of device operates like a single-electron memory latch.

Semiconductor quantumdots provide a two-dimensional analogy for real atoms and show promise for the implementation of scalable quantum computers. Here, we investigate the charge configurations in a silicon metal-oxide-semiconductor double quantumdot tunnel coupled to a single reservoir of electrons. By operating the system in the few-electron regime, the stability diagram shows hysteretic tunnelling events that depend on the history of the dots charge occupancy. We present a model which accounts for the observed hysteretic behaviour by extending the established description for transport in double dots coupled to two reservoirs. We demonstrate that this type of device operates like a single-electron memory latch.

We introduce a quantumdot in topological insulator nanofilm as a bump at the surface of the nanofilm. Such a quantumdot can localize an electron if the size of the dot is large enough, ≳5 nm. The quantumdot in topological insulator nanofilm has states of two types, which belong to two ('conduction' and 'valence') bands of the topological insulator nanofilm. We study the energy spectra of such defined quantumdots. We also consider intraband and interband optical transitions within the dot. The optical transitions of the two types have the same selection rules. While the interband absorption spectra have multi-peak structure, each of the intraband spectra has one strong peak and a few weak high frequency satellites. PMID:24590177

Recent advances in nanofabrication allow microstructured magnetic potentials to be applied to ballistic electrons in high mobility two-dimensional electron gases (2DEG). Electronic transport in quantum wires with single magnetic quantumdot was studied by some of present authors [1], where the magnetic quantumdot is defined by two different magnetic fields B and B0 inside and outside the circular region, respectively. It was shown that the conductance properties depend strongly on whether B^* is parallel or antiparallel to B_0. In this work, we investigate the conductance of quantum wires with magnetic quantumdots in series. The each magnetic quantumdot is defined in the same way as the single dot case. Conductance is calculated numerically by applying the recursive Green's function method based on the lattice Hamiltonian. Our numerical results show the conductance modulation due to the presence of new types of quasi-bound states formed around multiple magnetic quantumdots. [1]H.-S. Sim, G. Ihm, N. Kim, and K. J. Chang, Phys. Rev. Lett 87, 146601 (2001)

The widely popular class of quantum-dot molecular labels could so far not be utilized as standard fluorescent probes in STED (stimulated emission depletion) nanoscopy. This is because broad quantum-dot excitation spectra extend deeply into the spectral bands used for STED, thus compromising the transient fluorescence silencing required for attaining super-resolution. Here we report the discovery that STED nanoscopy of several red-emitting commercially available quantumdots is in fact successfully realized by the increasingly popular 775 nm STED laser light. A resolution of presently ~50 nm is demonstrated for single quantumdots, and sub-diffraction resolution is further shown for imaging of quantum-dot-labelled vimentin filaments in fibroblasts. The high quantum-dot photostability enables repeated STED recordings with >1,000 frames. In addition, we have evidence that the tendency of quantum-dot labels to blink is largely suppressed by combined action of excitation and STED beams. Quantum-dot STED significantly expands the realm of application of STED nanoscopy, and, given the high stability of these probes, holds promise for extended time-lapse imaging.

Colloidal nanocrystal quantumdots comprising an inner core having an average diameter of at least 1.5 nm and an outer shell, where said outer shell comprises multiple monolayers, wherein at least 30% of the quantumdots have an on-time fraction of 0.80 or greater under continuous excitation conditions for a period of time of at least 10 minutes.

The widely popular class of quantum-dot molecular labels could so far not be utilized as standard fluorescent probes in STED (stimulated emission depletion) nanoscopy. This is because broad quantum-dot excitation spectra extend deeply into the spectral bands used for STED, thus compromising the transient fluorescence silencing required for attaining super-resolution. Here we report the discovery that STED nanoscopy of several red-emitting commercially available quantumdots is in fact successfully realized by the increasingly popular 775 nm STED laser light. A resolution of presently ∼50 nm is demonstrated for single quantumdots, and sub-diffraction resolution is further shown for imaging of quantum-dot-labelled vimentin filaments in fibroblasts. The high quantum-dot photostability enables repeated STED recordings with >1,000 frames. In addition, we have evidence that the tendency of quantum-dot labels to blink is largely suppressed by combined action of excitation and STED beams. Quantum-dot STED significantly expands the realm of application of STED nanoscopy, and, given the high stability of these probes, holds promise for extended time-lapse imaging. PMID:25980788

Semiconductor quantumdots are quickly becoming a critical diagnostic tool for discerning cellular function at the molecular level. Their high brightness, long-lasting, size-tunable, and narrow luminescence set them apart from conventional fluorescence dyes. Quantumdots are being developed for a variety of biologically oriented applications, including fluorescent assays for drug discovery, disease detection, single protein tracking, and intracellular reporting. This review introduces the science behind quantumdots and describes how they are made biologically compatible. Several applications are also included, illustrating strategies toward target specificity, and are followed by a discussion on the limitations of quantumdot approaches. The article is concluded with a look at the future direction of quantumdots.

Semiconductor quantumdots are quickly becoming a critical diagnostic tool for discerning cellular function at the molecular level. Their high brightness, long-lasting, sizetunable, and narrow luminescence set them apart from conventional fluorescence dyes. Quantumdots are being developed for a variety of biologically oriented applications, including fluorescent assays for drug discovery, disease detection, single protein tracking, and intracellular reporting. This review introduces the science behind quantumdots and describes how they are made biologically compatible. Several applications are also included, illustrating strategies toward target specificity, and are followed by a discussion on the limitations of quantumdot approaches. The article is concluded with a look at the future direction of quantumdots. PMID:21276935

Modern technology is founded on the intimate understanding of how to utilize and control electrons. Next to electrons, nature uses phonons, quantized vibrations of an elastic structure, to carry energy, momentum and even information through solids. Phonons permeate the crystalline components of modern technology, yet in terms of technological utilization phonons are far from being on par with electrons. Here we demonstrate how phonons can be employed to render a single quantumdot pair optically transparent. This phonon-induced transparency is realized via the formation of a molecular polaron, the result of a Fano-type quantum interference, which proves that we have accomplished making typically incoherent and dissipative phonons behave in a coherent and non-dissipative manner. We find the transparency to be widely tunable by electronic and optical means. Thereby we show amplification of weakest coupling channels. We further outline the molecular polaron's potential as a control element in phononic circuitry architecture. PMID:24534815

We present a microscopic theory of optical initialization, control, and detection for a single electron spin in a quantumdot embedded into a zero-dimensional microcavity. The strong coupling regime of the trion and the cavity mode is addressed. We demonstrate that efficient spin orientation by a single circularly polarized pulse is possible in relatively weak transverse magnetic fields. The possibilities for spin control by additional circularly polarized pulse are analyzed. Under optimal conditions the Kerr and Faraday rotation angles induced by the spin polarized electron may reach tens of degrees.

A new model of refrigerator consisting of a spin-splitting quantumdot coupled with two ferromagnetic reservoirs and a ferromagnetic insulator is proposed. The rate equation is used to calculate the occupation probabilities of the quantumdot. The expressions of the electron and magnon currents are obtained. The region that the system can work in as a refrigerator is determined. The cooling power and coefficient of performance (COP) of the refrigerator are derived. The influences of the magnetic field, applied voltage, and polarization of two leads on the performance are discussed. The performances of two different magnon-driven quantumdot refrigerators are compared.

Fluorescent semiconductor quantumdots that can serve as "on/off" labels for bacteria and other living cells are undergoing development. The "on/off" characterization of these quantumdots refers to the fact that, when properly designed and manufactured, they do not fluoresce until and unless they come into contact with viable cells of biological species that one seeks to detect. In comparison with prior fluorescence-based means of detecting biological species, fluorescent quantumdots show promise for greater speed, less complexity, greater sensitivity, and greater selectivity for species of interest. There are numerous potential applications in medicine, environmental monitoring, and detection of bioterrorism.

Recent advancements in nanotechnology create a need for a better understanding of the underlying physical processes that lead to the different behavior of nanoscale structures in comparison to bulk materials. The influence of the surrounding environment on the physical and optical properties of nanoscale objects embedded inside them is of particular interest. This research is focused on the optical properties of semiconductor quantumdots which are zero-dimensional nanostructures. There are many investigation techniques for measuring the local parameters and structural characteristics of QuantumDot structures. They include X-ray diffraction, Transmission Electron Microscopy, Wavelength Dispersive Spectroscopy, etc. However, none of these is suitable for the study of large areas of quantumdots matrices and substrates. The existence of spatial inhomogeneity in the quantumdots allows for a deeper and better understanding of underlying physical processes responsible in particular for the observed changes in photoluminescence (PL) characteristics. Spectroscopic PL mapping can reveal areas of improved laser performance of InAs - InGaAs quantumdots structures. Establishing physical mechanisms responsible for two different types of spatial PL inhomogeneity in InAs/InGaAs quantumdots structures for laser applications was the first objective of this research. Most of the bio-applications of semiconductor quantumdots utilize their superior optical properties over organic fluorophores. Therefore, optimization of QD labeling performance with biomolecule attachment was another focus of this research. Semiconductor quantumdots suspended in liquids were investigated, especially the influence of surrounding molecules that may be attached or bio-conjugated to the quantumdots for specific use in biological reactions on the photoluminescence spectrum. Provision of underlying physical mechanisms of optical property instability of CdSe/ZnS quantumdots used for biological

Universal quantum logic gates are important elements for a quantum computer. In contrast to previous constructions on one degree of freedom (DOF) of quantum systems, we investigate the possibility of parallel quantum computations dependent on two DOFs of photon systems. We construct deterministic hyper-controlled-not (hyper-CNOT) gates operating on the spatial-mode and the polarization DOFs of two-photon or one-photon systems by exploring the giant optical circular birefringence induced by quantum-dot spins in one-sided optical microcavities. These hyper-CNOT gates show that the quantum states of two DOFs can be viewed as independent qubits without requiring auxiliary DOFs in theory. This result can reduce the quantum resources by half for quantum applications with large qubit systems, such as the quantum Shor algorithm.

Universal quantum logic gates are important elements for a quantum computer. In contrast to previous constructions on one degree of freedom (DOF) of quantum systems, we investigate the possibility of parallel quantum computations dependent on two DOFs of photon systems. We construct deterministic hyper-controlled-not (hyper-CNOT) gates operating on the spatial-mode and the polarization DOFs of two-photon or one-photon systems by exploring the giant optical circular birefringence induced by quantum-dot spins in one-sided optical microcavities. These hyper-CNOT gates show that the quantum states of two DOFs can be viewed as independent qubits without requiring auxiliary DOFs in theory. This result can reduce the quantum resources by half for quantum applications with large qubit systems, such as the quantum Shor algorithm. PMID:25030424

A diagnostics of infectious diseases can be done by the immunologic methods or by the amplification of nucleic acid specific to contagious agent using polymerase chain reaction. However, in transmissible spongiform encephalopathies, the infectious agent, prion protein (PrPSc), has the same sequence of nucleic acids as a naturally occurring protein. The other issue with the diagnosing based on the PrPSc detection is that the pathological form of prion protein is abundant only at late stages of the disease in a brain. Therefore, the diagnostics of prion protein caused diseases represent a sort of challenges as that hosts can incubate infectious prion proteins for many months or even years. Therefore, new in vivo assays for detection of prion proteins and for diagnosis of their relation to neurodegenerative diseases are summarized. Their applicability and future prospects in this field are discussed with particular aim at using quantumdots as fluorescent labels. PMID:24055838

Dimensionality is an important factor to govern the electronic structures of semiconductor nanocrystals. The quantum confinement energies in one-dimensional quantum wires and zero-dimensional quantumdots are quite different. Using large-scale first-principles calculations, we systematically study the electronic structures of semiconductor (including group IV, III-V, and II-VI) surface-passivated quantum wires and dots. The band-gap energies of quantum wires and dots have the same scaling with diameter for a given material. The ratio of band-gap-increases between quantum wires and dots is material-dependent, and slightly deviates from 0.586 predicted by effective-mass approximation. Highly linear polarization of photoluminescence in quantum wires is found. The degree of polarization decreases with the increasing temperature and size.

We theoretically analyze electronic spin transport through a triple quantumdot in series, attached to electrical contacts, where the drain contact is coupled to the central dot. We show that current rectification is observed in the device due to current blockade. The current blocking mechanism is originated by a destructive interference of the electronic wavefunction at the drain dot. There, the electrons are coherently trapped in a singlet two-electron dark state, which is a coherent superposition of the electronic wavefunction in the source dot and in the dot isolated from the contacts. Its formation gives rise to zero current and current rectification as the voltage is swept. We analyze this behavior analytically and numerically for both zero and finite magnetic dc fields. On top of that, we include phenomenologically a finite spin relaxation rate and calculate the current numerically. Our results show that triple dots in series can be designed to behave as quantum charge rectifiers. PMID:22442135

Currently there is strong interest in quantum information processing(See, for example, The Physics of Quantum Information, eds. D. Bouwmeester, A. Ekert and A. Zeilinger (Springer, Berlin, 2000).) in a solid state environment. Many approaches mimic atomic physics concepts in which semiconductor quantumdots are implemented as artificial atoms. An essential building block of a quantum processor is a gate which entangles the states of two quantum bits. Recently a pair of vertically aligned quantumdots has been suggested as optically driven quantum gate(P. Hawrylak, S. Fafard, and Z. R. Wasilewski, Cond. Matter News 7, 16 (1999).)(M. Bayer, P. Hawrylak, K. Hinzer, S. Fafard, M. Korkusinski, Z.R. Wasilewski, O. Stern, and A. Forchel, Science 291, 451 (2001).): The quantum bits are individual carriers either on dot zero or dot one. The different dot indices play the same role as a "spin", therefore we call them "isospin". Quantum mechanical tunneling between the dots rotates the isospin and leads to superposition of these states. The quantum gate is built when two different particles, an electron and a hole, are created optically. The two particles form entangled isospin states. Here we present spectrocsopic studies of single self-assembled InAs/GaAs quantumdot molecules that support the feasibility of this proposal. The evolution of the excitonic recombination spectrum with varying separation between the dots allows us to demonstrate coherent tunneling of carriers across the separating barrier and the formation of entangled exciton states: Due to the coupling between the dots the exciton states show a splitting that increases with decreasing barrier width. For barrier widths below 5 nm it exceeds the thermal energy at room temperature. For a given barrier width, we find only small variations of the tunneling induced splitting demonstrating a good homogeneity within a molecule ensemble. The entanglement may be controlled by application of electromagnetic field. For

We have carried out a series of ab-initio calculations to investigate changes in the optical properties of Si quantumdots as a function of surface passivation. In particular, we have compared hydrogen passivated dots with those having alkyl groups at the surface. We find that, while on clusters with reconstructed surfaces a complete alkyl passivation is possible, steric repulsion prevents full passivation of Si dots with unreconstructed surfaces. In addition, our calculations show that steric repulsion may have a dominant effect in determining the surface structure, and eventually the stability of alkyl passivated clusters, with results dependent on the length of the carbon chain. Alkyl passivation weakly affects optical gaps of silicon quantumdots, while it substantially decreases ionization potentials and electron affinities and affect their excited state properties. On the basis of our results we propose that alkyl terminated quantumdots may be size selected taking advantage of the change in ionization potential as a function of the cluster size.

We prepare a gate-defined quadruple quantumdot to study the gate-tunability of single to quadruple quantumdots with finite inter-dot tunnel couplings. The measured charging energies of various double dots suggest that the dot size is governed by the gate geometry. For the triple and quadruple dots, we study the gate-tunable inter-dot tunnel couplings. For the triple dot, we find that the effective tunnel coupling between side dots significantly depends on the alignment of the center dot potential. These results imply that the present quadruple dot has a gate performance relevant for implementing spin-based four-qubits with controllable exchange couplings.

The NASA Glenn Research Center has been investigating the synthesis of quantumdots of CdSe and CuInS2 for use in intermediate-bandgap solar cells. Using quantumdots in a solar cell to create an intermediate band will allow the harvesting of a much larger portion of the available solar spectrum. Theoretical studies predict a potential efficiency of 63.2 percent, which is approximately a factor of 2 better than any state-of-the-art devices available today. This technology is also applicable to thin-film devices--where it offers a potential four-fold increase in power-to-weight ratio over the state of the art. Intermediate-bandgap solar cells require that quantumdots be sandwiched in an intrinsic region between the photovoltaic solar cell's ordinary p- and n-type regions (see the preceding figure). The quantumdots form the intermediate band of discrete states that allow sub-bandgap energies to be absorbed. However, when the current is extracted, it is limited by the bandgap, not the individual photon energies. The energy states of the quantumdot can be controlled by controlling the size of the dot. Ironically, the ground-state energy levels are inversely proportional to the size of the quantumdots. We have prepared a variety of quantumdots using the typical organometallic synthesis routes pioneered by Ba Wendi et al., in the early 1990's. The most studied quantumdots prepared by this method have been of CdSe. To produce these dots, researchers inject a syringe of the desired organometallic precursors into heated triocytlphosphine oxide (TOPO) that has been vigorously stirred under an inert atmosphere (see the following figure). The solution immediately begins to change from colorless to yellow, then orange and red/brown, as the quantumdots increase in size. When the desired size is reached, the heat is removed from the flask. Quantumdots of different sizes can be identified by placing them under a "black light" and observing the various color differences in

Yeast, bacteria and fungi have been used to synthesize a variety of nanocrystals. Now, the metal detoxification process in the gut of an earthworm is exploited to produce biocompatible cadmium telluride quantumdots.

A method has been developed for inserting submonolayer (SML) quantumdots (QDs) or SML QD stacks, instead of conventional Stranski-Krastanov (S-K) QDs, into the active region of intersubband photodetectors. A typical configuration would be InAs SML QDs embedded in thin layers of GaAs, surrounded by AlGaAs barriers. Here, the GaAs and the AlGaAs have nearly the same lattice constant, while InAs has a larger lattice constant. In QD infrared photodetector, the important quantization directions are in the plane perpendicular to the normal incidence radiation. In-plane quantization is what enables the absorption of normal incidence radiation. The height of the S-K QD controls the positions of the quantized energy levels, but is not critically important to the desired normal incidence absorption properties. The SML QD or SML QD stack configurations give more control of the structure grown, retains normal incidence absorption properties, and decreases the strain build-up to allow thicker active layers for higher quantum efficiency.

We report on strong renormalization encountered in periodically driven interacting quantumdots in the nonadiabatic regime. Correlations between lead and dot electrons enhance or suppress the amplitude of driving depending on the sign of the interaction. Employing a newly developed flexible renormalization-group-based approach for periodic driving to an interacting resonant level we show analytically that the magnitude of this effect follows a power law. Our setup can act as a non-Markovian, single-parameter quantum pump. PMID:26824557

A charge patching method and an idealized surface passivation are used to calculate the single electronic states of IV-IV, III-V, II-VI semiconductor quantumdots up to a thousand atoms. This approach scales linearly and has a 1000 fold speed-up compared to direct first principle methods with a cost of eigen energy error of about 20 meV. The calculated quantumdot band gaps are parametrized for future references.

An electron spin confined to a semiconductor quantumdot is not subject to the classical spin relaxation mechanisms known for free carriers but it strongly interacts with the nuclear spin system via the hyperfine interaction. We show in time resolved photoluminescence spectroscopy experiments on ensembles of self assembled InAs quantumdots in GaAs that this interaction leads to strong electron spin dephasing.

This thesis describes experiments on quantumdots made by locally gating one-dimensional quantum wires. The first experiment studies a double quantumdot device formed in a Ge/Si core/shell nanowire. In addition to measuring transport through the double dot, we detect changes in the charge occupancy of the double dot by capacitively coupling it to a third quantumdot on a separate nanowire using a floating gate. We demonstrate tunable tunnel coupling of the double dot and quantify the strength of the tunneling using the charge sensor. The second set of experiments concerns carbon nanotube double quantumdots. In the first nanotube experiment, spin-dependent transport through the double dot is compared in two sets of devices. The first set is made with carbon containing the natural abundance of 12C (99%) and 13C (1%), the second set with the 99% 13C and 1% 12C. In the devices with predominantly 13C, we find evidence in spin-dependent transport of the interaction between the electron spins and the 13C nuclear spins that was much stronger than expected and not present in the 12C devices. In the second nanotube experiment, pulsed gate experiments are used to measure the timescales of spin relaxation and dephasing in a two-electron double quantumdot. The relaxation time is longest at zero magnetic field and goes through a minimum at higher field, consistent with the spin-orbit-modified electronic spectrum of carbon nanotubes. We measure a short dephasing time consistent with the anomalously strong electron-nuclear interaction inferred from the first nanotube experiment.

The two-photon bound-bound (TPBB) and the two-photon bound-free (TPBF) processes are studied for the electron in the initial 1S state in the conduction band of the ZnO quantumdot (QD) embedded in the HfO2 and the AlN matrices. The energy and the wave functions of the QD are obtained by using the effective mass approximation with a finite barrier height at the dot-matrix interface. Using the second order perturbation theory results are obtained for the two-photon absorption coefficient and the photoelectric cross section. The photoelectric cross section ratio for the circularly to the linearly polarized photons is also obtained. It is observed that the two-photon processes depend significantly on the polarization of the incident beam, the dot size, and the surrounding matrix. It is found that the electric quadrupole interaction enhance the TPBF photoelectric cross section.

Semiconductor quantumdots are ideal candidates for quantum information applications in solid-state technology. However, advanced theoretical and experimental tools are required to coherently control, for example, the electronic charge in these systems. Here we demonstrate how quantum optimal control theory provides a powerful way to manipulate the electronic structure of coupled quantumdots with an extremely high fidelity. As alternative control fields we apply both laser pulses as well as electric gates, respectively. We focus on double and triple quantumdots containing a single electron or two electrons interacting via Coulomb repulsion. In the two-electron situation we also briefly demonstrate the challenges of timedependent density-functional theory within the adiabatic local-density approximation to produce comparable results with the numerically exact approach.

We consider a triple quantumdot system in a triangular geometry with one of the dots connected to metallic leads. We investigate quantum phase transition between local moment phase and Kondo screened strong coupling phase in triple quantumdots where energy levels of dots are deviated from the particle-hole symmetric point. The effect of on-site energy of dots on quantum phase transition between local moment phase and Kondo screened strong coupling phase in triple quantumdots is studied based on the analytical arguments and the numerical renormalization group method. The results show that the critical value of tunnel coupling between side dots decreases when the energy level of embedded dot rises up from the symmetric point to the Fermi level and the critical value increases when the energy levels of two side dots rise up. The study of the influence of on-site-energy changes on the quantum phase transitions in triple quantumdots has the importance for clarifying the mechanism of Kondo screening in triple quantumdots where energy levels of dots are deviated from the particle-hole symmetric point.

Heteroepitaxy-atomically aligned growth of a crystalline film atop a different crystalline substrate-is the basis of electrically driven lasers, multijunction solar cells, and blue-light-emitting diodes. Crystalline coherence is preserved even when atomic identity is modulated, a fact that is the critical enabler of quantum wells, wires, and dots. The interfacial quality achieved as a result of heteroepitaxial growth allows new combinations of materials with complementary properties, which enables the design and realization of functionalities that are not available in the single-phase constituents. Here we show that organohalide perovskites and preformed colloidal quantumdots, combined in the solution phase, produce epitaxially aligned 'dots-in-a-matrix' crystals. Using transmission electron microscopy and electron diffraction, we reveal heterocrystals as large as about 60 nanometres and containing at least 20 mutually aligned dots that inherit the crystalline orientation of the perovskite matrix. The heterocrystals exhibit remarkable optoelectronic properties that are traceable to their atom-scale crystalline coherence: photoelectrons and holes generated in the larger-bandgap perovskites are transferred with 80% efficiency to become excitons in the quantumdot nanocrystals, which exploit the excellent photocarrier diffusion of perovskites to produce bright-light emission from infrared-bandgap quantum-tuned materials. By combining the electrical transport properties of the perovskite matrix with the high radiative efficiency of the quantumdots, we engineer a new platform to advance solution-processed infrared optoelectronics. PMID:26178963

Heteroepitaxy--atomically aligned growth of a crystalline film atop a different crystalline substrate--is the basis of electrically driven lasers, multijunction solar cells, and blue-light-emitting diodes. Crystalline coherence is preserved even when atomic identity is modulated, a fact that is the critical enabler of quantum wells, wires, and dots. The interfacial quality achieved as a result of heteroepitaxial growth allows new combinations of materials with complementary properties, which enables the design and realization of functionalities that are not available in the single-phase constituents. Here we show that organohalide perovskites and preformed colloidal quantumdots, combined in the solution phase, produce epitaxially aligned `dots-in-a-matrix' crystals. Using transmission electron microscopy and electron diffraction, we reveal heterocrystals as large as about 60 nanometres and containing at least 20 mutually aligned dots that inherit the crystalline orientation of the perovskite matrix. The heterocrystals exhibit remarkable optoelectronic properties that are traceable to their atom-scale crystalline coherence: photoelectrons and holes generated in the larger-bandgap perovskites are transferred with 80% efficiency to become excitons in the quantumdot nanocrystals, which exploit the excellent photocarrier diffusion of perovskites to produce bright-light emission from infrared-bandgap quantum-tuned materials. By combining the electrical transport properties of the perovskite matrix with the high radiative efficiency of the quantumdots, we engineer a new platform to advance solution-processed infrared optoelectronics.

Highly luminescent semiconductor quantumdots have been covalently coupled to biological molecules for use in ultrasensitive biological detection. This new class of luminescent labels is considerably brighter and more resistant againt photobleaching in comparison with organic dyes. Quantumdots labeled with the protein transferrin undergo receptor-mediated endocytosis (RME) in cultured HeLa cells, and those dots that were conjugated to immunomolecules recognize specific antibodies or antigens. In addition, we show that DNA functionalized quantumdots can be used to target specific genes by hybridization. We expect that quantumdot bioconjugates will have a broad range of biological applications, such as ligand-receptor interactions, real-time monitoring of molecular trafficking inside living cells, multicolor fluorescence in-situ hybridization (FISH), high-sensitivity detection in miniaturized devices (e.g., DNA chips), and fluorescent tagging of combinatorial chemical libraries. A potential clinical application is the use of quantumdots for ultrasensitive viral RNA detection, in which as low as 100 copies of hepatitis C and HIV viruses per ml blood should be detected.

In this paper we show the characterization of two kinds of quantumdots: hydrophilic and hydrophobic, with core and core/shell respectively, using spectroscopy techniques such as UV-Vis, fluorescence and Raman. We determined the quantum yield in the quantumdots using the quinine sulphate as standard. This salt is commonly used because of its quantum yield (56%) and stability. For the CdTe excitation, we used a wavelength of 549nm and for the CdSe/ZnS excitation a wavelength of 527nm. The results show that CdSe/ZnS (49%) has better fluorescence, better quantumdots, and confirm the fluorescence result. The quantumdots have shown a good fluorescence performance, so this property will be used to replace dyes, with the advantage that quantumdots are less toxic than some dyes like the rhodamine. In addition, in this work we show different techniques to find the quantumdots emission: fluorescence spectrum, synchronous spectrum and Raman spectrum.

(In,Mn)As multilayer quantumdots structures were grown by molecular beam epitaxy using a Mn selective doping of the central parts of quantumdots. The study of the structural and magneto-optical properties of the samples with three and five layers of (In,Mn)As quantumdots has shown that during the quantumdots assembly, the out-diffusion of Mn from the layers with (In,Mn)As quantumdots can occur resulting in the formation of the extended defects. To produce a high quality structures using the elaborated technique of selective doping, the number of (In,Mn)As quantumdot layers should not exceed three.

For the past fifteen years, single semiconductor quantumdots, often referred to as solid-state artificial atoms, have been at the forefront of various research direction lines for experimental quantum information science, in particular in the development of practical sources of quantum states of light. Here we review the research to date, on the tailoring of the emission properties from single quantumdots producing single photons, indistinguishable single photons and entangled photon pairs. Finally, the progress and future prospects for applications of single dots in quantum information processing is considered.

The effect of n-type doping on the luminescence properties of II-VI quantumdots is studied. The addition of two shells of CdS on CdSe quantumdots prevents the creation of surface traps and makes the system stable under reducing environment. The injection of electrons into films of quantumdots leads to lower photoluminescence (PL) efficiency, with the extent of quenching dependent on both the number and the quantum states of the spectator charges in the nanocrystal. It is found that a 1Pe electron is an eightfold better PL quencher than the 1Se electron. Reduced threshold for stimulated emission is also observed in doped CdSe/CdS films. Time resolved photoluminescence measurements are used to extract the recombination rates of a charged exciton, called trion. It is observed that the negative trion has a radiative rate ˜2.2 +/- 0.4x faster than a neutral exciton, while its non-radiative recombination rate is slower than the biexciton non-radiative recombination rate by a factor of 7.5 +/- 1.7. The knowledge of the recombination rates of the trion enables us to calculate the quantum yield of a negative trion to be ˜10% for the nanocrystals investigated in our work. This is larger than the off state quantum yield from a single quantumdot photoluminescence trajectory and eliminates the formation of negative trion as the possible reason for the PL blinking of single quantumdots. Single quantumdot electrochemistry has also been achieved. It is shown that by varying the Fermi level of the system electrons can be reversibly injected into and extracted out of single CdSe/CdS and CdSe/ZnS nanoparticles to modulate the photoluminescence.

Spins in charged semiconductor quantumdots are currently generating much interest, both from a fundamental physics standpoint, as well as for their potential technological relevance. Being naturally a two-level quantum system, each of these spins can encode a bit of quantum information. Optically controlled spins in quantumdots possess several desirable properties: their spin coherence times are long, they allow for all-optical manipulation---which translates into fast logic gates---and their coupling to photons offers a straightforward route to exchange of quantum information between spatially separated sites. Designing the laser fields to achieve the unprecedented amount of control required for quantum information tasks is a challenging goal, towards which there has been recent progress. Special properties of hyperbolic secant optical pulses enabled the design of single qubit rotations, initially developed about the growth axis z [1], and later about an arbitrary direction [2]. Recently we demonstrated our theoretical proposal [1] in an ensemble of InAs/GaAs quantumdots by implementing ultrafast rotations about the z axis by an arbitrary angle [3], with the angle of rotation as a function of the optical detuning in excellent agreement with the theoretical prediction. We also developed two-qubit conditional control in a quantumdot `molecule' using the electron-hole exchange interaction [4]. In addition to its importance in quantumdot-based quantum computation, our two-qubit gate can also play an important role in photonic cluster state generation for measurement-based quantum computing [5]. [1] S. E. Economou, L. J. Sham, Y. Wu, D. S. Steel, Phys. Rev. 74, 205415 (2006) [2] S. E. Economou and T. L. Reinecke, Phys. Rev. Lett., 99, 217401 (2007) [3] A. Greilich, S. E. Economou et al, Nature Phys. 5, 262 (2009) [4] S. E. Economou and T. L. Reinecke, Phys. Rev. B, 78, 115306 (2008) [5] S. E. Economou, N. H. Lindner, and T. Rudolph, in preparation

We review a microscopic laser theory for quantumdots as active material for quantum cascade lasers, in which carrier collisions are treated at the level of quantum kinetic equations. The computed characteristics of such a quantum-dot active material are compared to a state-of-the-art quantum-well quantum cascade laser. We find that the current requirement to achieve a comparable gain-length product is reduced compared to that of the quantum-well quantum cascade laser.

A system of two or more quantumdots interacting with a dissipative plasmonic nanostructure is investigated in detail by using a cavity quantum electrodynamics approach with a model Hamiltonian. We focus on determining and understanding system configurations that generate multiple bipartite quantum entanglements between the occupation states of the quantumdots. These configurations include allowing for the quantumdots to be asymmetrically coupled to the plasmonic system. Analytical solution of a simplified limit for an arbitrary number of quantumdots and numerical simulations and optimization for the two- and three-dot cases are used to develop guidelines for maximizing the bipartite entanglements. For any number of quantumdots, we show that through simple starting states and parameter guidelines, one quantumdot can be made to share a strong amount of bipartite entanglement with all other quantumdots in the system, while entangling all other pairs to a lesser degree.

Dot-in-well (DWELL) quantum-dot infrared photodetectors (QDIPs) [DWELL-QDIPs] are subjects of research as potentially superior alternatives to prior QDIPs. Heretofore, there has not existed a reliable method for fabricating quantumdots (QDs) having precise, repeatable dimensions. This lack has constituted an obstacle to the development of uniform, high-performance, wavelength-tailorable QDIPs and of focal-plane arrays (FPAs) of such QDIPs. However, techniques for fabricating quantum-well infrared photodetectors (QWIPs) having multiple-quantum- well (MQW) structures are now well established. In the present research on DWELL-QDIPs, the arts of fabrication of QDs and QWIPs are combined with a view toward overcoming the deficiencies of prior QDIPs. The longer-term goal is to develop focal-plane arrays of radiationhard, highly uniform arrays of QDIPs that would exhibit high performance at wavelengths from 8 to 15 m when operated at temperatures between 150 and 200 K. Increasing quantum efficiency is the key to the development of competitive QDIP-based FPAs. Quantum efficiency can be increased by increasing the density of QDs and by enhancing infrared absorption in QD-containing material. QDIPs demonstrated thus far have consisted, variously, of InAs islands on GaAs or InAs islands in InGaAs/GaAs wells. These QDIPs have exhibited low quantum efficiencies because the numbers of QD layers (and, hence, the areal densities of QDs) have been small typically five layers in each QDIP. The number of QD layers in such a device must be thus limited to prevent the aggregation of strain in the InAs/InGaAs/GaAs non-lattice- matched material system. The approach being followed in the DWELL-QDIP research is to embed In- GaAs QDs in GaAs/AlGaAs multi-quantum- well (MQW) structures (see figure). This material system can accommodate a large number of QD layers without excessive lattice-mismatch strain and the associated degradation of photodetection properties. Hence, this material

Quantum state joining has been recently experimentally demonstrated [C. Vitelli et al., Nat. Photon. 7, 521 (2013), 10.1038/nphoton.2013.107] which can transfer two input photonic qubits into a photonic ququart. Here, we revisit these processes from a hybrid point of view. By exploring the giant optical circular birefringence induced by quantum-dot spins in one-sided optical microcavities, we introduce some deterministic joining schemes including two quantum-dot spin joining, hybrid photon and quantum-dot spin joining, and two-photon joining. The input quantum information is represented by one photon with polarization and spatial mode degrees of freedom (DOFs). These schemes are also adapted to the inverse processes called quantum state splitting because all the joining procedures are unitary and do not require projection and feed-forward steps. The fused photon is convenient for realizing elementary logic gates such as the controlled-not (cnot) gate, swap gate, and Toffoli gate. These hybrid fusion and splitting schemes provide flexible synthesis of the quantum-dot spin and photon in quantum applications. The transmission superiority of photons and storage superiority of the quantum-dot spin may be combined for quantum network communication or quantum computations.

In this paper by solving Dirac equation, we present an analytical solution to calculate energy levels and wave functions of mono- and bilayer graphene quantumdots. By supposing circularquantumdots, we solve Dirac equation and obtain energy levels and band gap with relations in a new closed and practical form. The energy levels are correlated with a radial quantum number and radius of quantumdots. In addition to monolayer quantumdots, AA- and AB-stacked bilayer quantumdots are investigated and their energy levels and band gap are calculated as well. Also, we analyze the influence of the quantumdots size on their energy spectrum. It can be observed that the band gap decreases as quantumdots' radius increases. On the other hand, increase in the band gap is more in AB-stacked bilayer quantumdots. Using the obtained relations, the band gap is obtained in each state. Comparing the energy spectra obtained from the tight-binding approximation with those of our obtained relations shows that the behavior of the energies as function of the dot size is qualitatively similar, but in some cases, quantitative differences can be seen. As quantumdots radius increases, the analytical results approach to the tight-binding method results.

Motivated by recent interest in implementing circuit quantum electrodynamics with semiconducting quantumdots, we study charge transfer through a double quantumdot (DQD) capacitively coupled to a superconducting cavity subject to a microwave field. We analyze the DQD current response using input-output theory and determine the optimal parameter regime for complete absorption of radiation and efficient conversion of microwave photons to electric current. For experimentally available DQD systems, we show that the cavity-coupled DQD operates as a photon-to-charge converter with quantum efficiencies up to 80% C.W. acknowledges support by the Intelligence Community Postdoctoral Research Fellowship Program.

We report on the existence of a bound state in the continuum (BIC) of quantum rods (QR). QRs are novel elongated InGaAs quantumdot nanostructures embedded in the shallower InGaAs quantum well. BIC appears as an excited confined dot state and energetically above the bottom of a well subband continuum. We prove that high height-to-diameter QR aspect ratio and the presence of a quantum well are indispensable conditions for accommodating the BIC. QRs are unique semiconductor nanostructures, exhibiting this mathematical curiosity predicted 83 years ago by Wigner and von Neumann.

We report a theoretical investigation on the energy spectrum and the effective Zeeman energy (EZE) in monolayer MoS2 circularquantumdots, subjected to an out-of-plane magnetic field. Interestingly, we observe the emergence of energy-locked modes, depending on the competition between the dot confinement and the applied magnetic field, for either the highest K-valley valence band or the lowest [Formula: see text]-valley conduction band. Moreover, an unusual dot-size-independent EZE behavior of the highest valence and the lowest conduction bands is found. Although the EZEs are insensitive to the variation of quantum confinement, both of them grow linearly with the magnetic field, similar to that in the monolayer MoS2 material. The EZEs along with their 'robustness' against dot confinements open opportunities of a universal magnetic control over the valley degree of freedom, for quantumdots of all sizes. PMID:27421077

We report a theoretical investigation on the energy spectrum and the effective Zeeman energy (EZE) in monolayer MoS2 circularquantumdots, subjected to an out-of-plane magnetic field. Interestingly, we observe the emergence of energy-locked modes, depending on the competition between the dot confinement and the applied magnetic field, for either the highest K-valley valence band or the lowest {{K}\\prime} -valley conduction band. Moreover, an unusual dot-size-independent EZE behavior of the highest valence and the lowest conduction bands is found. Although the EZEs are insensitive to the variation of quantum confinement, both of them grow linearly with the magnetic field, similar to that in the monolayer MoS2 material. The EZEs along with their ‘robustness’ against dot confinements open opportunities of a universal magnetic control over the valley degree of freedom, for quantumdots of all sizes.

Provided are methods of surface treatment of nanocrystal quantumdots after film deposition so as to exchange the native ligands of the quantumdots for exchange ligands that result in improvement in charge extraction from the nanocrystals.

A quantum bit computing architecture includes a plurality of single spin memory donor atoms embedded in a semiconductor layer, a plurality of quantumdots arranged with the semiconductor layer and aligned with the donor atoms, wherein a first voltage applied across at least one pair of the aligned quantumdot and donor atom controls a donor-quantumdot coupling. A method of performing quantum computing in a scalable architecture quantum computing apparatus includes arranging a pattern of single spin memory donor atoms in a semiconductor layer, forming a plurality of quantumdots arranged with the semiconductor layer and aligned with the donor atoms, applying a first voltage across at least one aligned pair of a quantumdot and donor atom to control a donor-quantumdot coupling, and applying a second voltage between one or more quantumdots to control a Heisenberg exchange J coupling between quantumdots and to cause transport of a single spin polarized electron between quantumdots.

This thesis presents experiments exploring the interactions of electron spins with electric fields in devices of up to four quantumdots. These experiments are particularly motivated by the prospect of using electric fields to control spin qubits. A novel hyperfine effect on a single spin in a quantumdot is presented in Chapter 2. Fluctuations of the nuclear polarization allow single-spin resonance to be driven by an oscillating electric field. Spin resonance spectroscopy revealed a nuclear polarization built up inside the quantumdot device by driving the resonance. The evolution of two coupled spins is controlled by the combination of hyperfine interaction, which tends to cause spin dephasing, and exchange, which tends to prevent it. In Chapter 3, dephasing is studied in a device with tunable exchange, probing the crossover between exchange-dominated and hyperfine-dominated regimes. In agreement with theoretical predictions, oscillations of the spin conversion probability and saturation of dephasing are observed. Chapter 4 deals with a three-dot device, suggested as a potential qubit controlled entirely by exchange. Preparation and readout of the qubit state are demonstrated, together with one out of two coherent exchange operations needed for arbitrary manipulations. A new readout technique allowing rapid device measurement is described. In Chapter 5, an attempt to make a two-qubit gate using a four-dot device is presented. Although spin qubit operation has not yet been possible, the electrostatic interaction between pairs of dots was measured to be sufficient in principle for coherent qubit coupling.

In this paper, we compute the tunneling of electrons in a nonvolatile quantumdot memory (NVQDM) cell during the WRITE operation. The transition rate of electrons from a quantum well channel to the quantumdots forming the floating gate is calculated using a recently reported method by Chuang et al.[1]. Tunneling current is computed based on transport of electrons from the channel to the floating quantumdots. The maximum number of electrons on a dot is calculated using surface electric field and break down voltage of the tunneling dielectric material. Comparison of tunneling for silicon oxide and high-k dielectric gate insulators is also described. Capacitance-Voltage characteristics of a NVQDM device are calculated by solving the Schrodinger and Poisson equations self-consistently. In addition, the READ operation of the memory has been investigated analytically. Results for 70 nm channel length Si NVQDMs are presented. Threshold voltage is calculated including the effect of the charge on nanocrystal quantumdots. Current-voltage characteristics are obtained using BSIM3v3 model [2-3]. This work is supported by Office of Navel Research (N00014210883, Dr. D. Purdy, Program Monitor), Connecticut Innovations Inc./TranSwitch (CII # 00Y17), and National Science Foundation (CCR-0210428) grants. [1] S. L. Chuang and N. Holonyak, Appl. Phys. Lett., 80, pp. 1270, 2002. [2] Y. Chen et. al., BSIM3v3 Manual, Elect. Eng. and Comp. Dept., U. California, Berkeley, CA, 1996. [3] W. Liu, MOSFET Models for SPICE Simulation, John Wiley & Sons, Inc., 2001.

We study the potential for controlled population transfer between the ground states of two anharmonic coupled quantumdots. We propose a method based on the interaction of the quantumdot structure with external electromagnetic fields. The interaction of the quantumdot system with the electromagnetic fields is studied with the use of the time-dependent Schroedinger equation. We present numerical results for an asymmetric quantumdot structure.

Like modern microprocessors today, future processors of quantum information may be implemented using all-electrical control of silicon-based devices. A semiconductor spin qubit may be controlled without the use of magnetic fields by using three electrons in three tunnel-coupled quantumdots. Triple dots have previously been implemented in GaAs, but this material suffers from intrinsic nuclear magnetic noise. Reduction of this noise is possible by fabricating devices using isotopically purified silicon. We demonstrate universal coherent control of a triple-quantum-dot qubit implemented in an isotopically enhanced Si/SiGe heterostructure. Composite pulses are used to implement spin-echo type sequences, and differential charge sensing enables single-shot state readout. These experiments demonstrate sufficient control with sufficiently low noise to enable the long pulse sequences required for exchange-only two-qubit logic and randomized benchmarking. PMID:26601186

Like modern microprocessors today, future processors of quantum information may be implemented using all-electrical control of silicon-based devices. A semiconductor spin qubit may be controlled without the use of magnetic fields by using three electrons in three tunnel-coupled quantumdots. Triple dots have previously been implemented in GaAs, but this material suffers from intrinsic nuclear magnetic noise. Reduction of this noise is possible by fabricating devices using isotopically purified silicon. We demonstrate universal coherent control of a triple-quantum-dot qubit implemented in an isotopically enhanced Si/SiGe heterostructure. Composite pulses are used to implement spin-echo type sequences, and differential charge sensing enables single-shot state readout. These experiments demonstrate sufficient control with sufficiently low noise to enable the long pulse sequences required for exchange-only two-qubit logic and randomized benchmarking. PMID:26601186

Numerical analysis of the simplest odd-numbered system of coupled quantumdots reveals an interplay between magnetic ordering, charge fluctuations, and the tendency of itinerant electrons in the leads to screen magnetic moments. The transition from local-moment to molecular-orbital behavior is visible in the evolution of correlation functions as the interdot coupling is increased. Resulting Kondo phases are presented in a phase diagram which can be sampled by measuring the zero-bias conductance. We discuss the origin of the even-odd effects by comparing with the double quantumdot.

We electrically measure intrinsic silicon quantumdots with electrostatically defined tunnel barriers. The presence of both p- and n-type ohmic contacts enables the accumulation of either electrons or holes. Thus, we are able to study both transport regimes within the same device. We investigate the effect of the tunnel barriers and the electrostatically defined quantumdots. There is greater localisation of charge states under the tunnel barriers in the case of hole conduction, leading to higher charge noise in the p-type regime.

We investigate the application of nanoscale topgates on exfoliated bilayer graphene to define quantumdot devices. At temperatures below 500 mK, the conductance underneath the grounded gates is suppressed, which we attribute to nearest neighbour hopping and strain-induced piezoelectric fields. The gate-layout can thus be used to define resistive regions by tuning into the corresponding temperature range. We use this method to define a quantumdot structure in bilayer graphene showing Coulomb blockade oscillations consistent with the gate layout.

Small clusters of two to three silica-coated nanocrystals coupled to plasmonic gap-bar antennas can exhibit photon antibunching, a characteristic of single quantum emitters. Through a detailed analysis of their photoluminescence emissions characteristics, it is shown that the observed photon antibunching is the evidence of coupled quantumdot formation resulting from the plasmonic enhancement of dipole-dipole interaction. PMID:26140499

In this paper we analyse the electronic properties of Dirac electrons in finite-size ribbons and in circular and hexagonal quantumdots. We show that due to the formation of sub-bands in the ribbons it is possible to spatially localize some of the electronic modes using a p-n-p junction. We also show that scattering of confined Dirac electrons in a narrow channel by an infinitely massive wall induces mode mixing, giving a qualitative reason for the fact that an analytical solution to the spectrum of Dirac electrons confined in a square box has not yet been found. A first attempt to solve this problem is presented. We find that only the trivial case k = 0 has a solution that does not require the existence of evanescent modes. We also study the spectrum of quantumdots of graphene in a perpendicular magnetic field. This problem is studied in the Dirac approximation, and its solution requires a numerical method whose details are given. The formation of Landau levels in the dot is discussed. The inclusion of the Coulomb interaction among the electrons is considered at the self-consistent Hartree level, taking into account the interaction with an image charge density necessary to keep the back-gate electrode at zero potential. The effect of a radial confining potential is discussed. The density of states of circular and hexagonal quantumdots, described by the full tight-binding model, is studied using the Lanczos algorithm. This is necessary to access the detailed shape of the density of states close to the Dirac point when one studies large systems. Our study reveals that zero-energy edge states are also present in graphene quantumdots. Our results are relevant for experimental research in graphene nanostructures. The style of writing is pedagogical, in the hope that newcomers to the subject will find this paper a good starting point for their research.

Cavity quantum electrodynamics techniques have turned out to be instrumental to probe or manipulate the electronic states of nanoscale circuits. Recently, cavity QED architectures have been extended to quantumdot circuits. These circuits are appealing since other degrees of freedom than the traditional ones (e.g. those of superconducting circuits) can be investigated. I will show how one can use carbon nanotube based quantumdots in that context. In particular, I will focus on the coherent coupling of a single spin or non-local Cooper pairs to cavity photons. Quantumdots also exhibit a wide variety of many body phenomena. The cQED architecture could also be instrumental for understanding them. One of the most paradigmatic phenomenon is the Kondo effect which is at the heart of many electron correlation effects. I will show that a cQED architecture has allowed us to observe the decoupling of spin and charge excitations in a Kondo system.

Single quantumdots (QDs) have great potential as building blocks for quantum information processing devices. However, one of the major difficulties in the fabrication of such devices is the placement of a single dot at a pre-determined position in the device structure, for example, in the centre of a photonic cavity. In this article we review some recent investigations in the site-controlled growth of InAs QDs on GaAs by molecular beam epitaxy. The method we use is ex-situ patterning of the GaAs substrate by electron beam lithography and conventional wet or dry etching techniques to form shallow pits in the surface which then determine the nucleation site of an InAs dot. This method is easily scalable and can be incorporated with marker structures to enable simple post-growth lithographic alignment of devices to each site-controlled dot. We demonstrate good site-control for arrays with up to 10 micron spacing between patterned sites, with no dots nucleating between the sites. We discuss the mechanism and the effect of pattern size, InAs deposition amount and growth conditions on this site-control method. Finally we discuss the photoluminescence from these dots and highlight the remaining challenges for this technique. To cite this article: P. Atkinson et al., C. R. Physique 9 (2008).

The charge and spin degrees of freedom of an electron constitute natural bases for constructing quantum two level systems, or qubits, in semiconductor quantumdots. The quantumdot charge qubit offers a simple architecture and high-speed operation, but generally suffers from fast dephasing due to strong coupling of the environment to the electron's charge. On the other hand, quantumdot spin qubits have demonstrated long coherence times, but their manipulation is often slower than desired for important future applications. This talk will present experimental progress of a `hybrid' qubit, formed by three electrons in a Si/SiGe double quantumdot, which combines desirable characteristics (speed and coherence) in the past found separately in qubits based on either charge or spin degrees of freedom. Using resonant microwaves, we first discuss qubit operations near the `sweet spot' for charge qubit operation. Along with fast (>GHz) manipulation rates for any rotation axis on the Bloch sphere, we implement two independent tomographic characterization schemes in the charge qubit regime: traditional quantum process tomography (QPT) and gate set tomography (GST). We also present resonant qubit operations of the hybrid qubit performed on the same device, DC pulsed gate operations of which were recently demonstrated. We demonstrate three-axis control and the implementation of dynamic decoupling pulse sequences. Performing QPT on the hybrid qubit, we show that AC gating yields π rotation process fidelities higher than 93% for X-axis and 96% for Z-axis rotations, which demonstrates efficient quantum control of semiconductor qubits using resonant microwaves. We discuss a path forward for achieving fidelities better than the threshold for quantum error correction using surface codes. This work was supported in part by ARO (W911NF-12-0607), NSF (PHY-1104660), DOE (DE-FG02-03ER46028), and by the Laboratory Directed Research and Development program at Sandia National Laboratories

Developing on-chip, dynamically reconfigurable visible lasers that can be integrated with additional optical and electronic components will enable adaptive optical components. In the present work, we demonstrate a reconfigurable quantumdot laser based on an integrated silica ultra high-Q microcavity. By attaching the quantumdot using a reversible, non-destructive bioconjugation process, the ability to remove and replace it with an alternative quantumdot without damaging the underlying microcavity device has been demonstrated. As a result of the absorption/emission characteristics of quantumdots, the same laser source can be used to excite quantumdots with distinct emission wavelengths.

CdSe QuantumDots (QDs) of size 0.85 nm were synthesized using chemical route. ZnO based QuantumDot Sensitized Solar Cell (QDSSC) was fabricated using CdSe QDs as sensitizer. The Pre-synthesized QDs were found to be successfully adsorbed on front ZnO electrode and had potential to replace organic dyes in Dye Sensitized Solar Cells (DSSCs). The efficiency of QDSSC was obtained to be 2.06 % at AM 1.5.

In "New Small QuantumDots for Neuroscience," Paul Selvin (University of Illinois, Urbana-Champaign) notes how the details of synapsis activity in the brain involves chemical receptors that facilitate the creation of the electrical connection between two nerves. In order to understand the details of this neuroscience phenomenon you need to be able to "see" what is happening at the scale of these receptors, which is around 10 nanometers. This is smaller than the diffraction limit of normal microscopy and it takes place on a 3 dimensional structure. Selvin describes the development of small quantumdots (on the order of 6-9 microns) that are surface-sensitized to interact with the receptors. This allows the application of photo-activated localized microscopy (PALM), a superresolution microscopy that can be scanned through focus to develop a 3D map on a scale that is the same size as the emitter, which in this case are the small quantumdots. The quantumdots are stable in time and provide access to the receptors which allows the imaging of the interactions taking place at the synoptic level.

An improved process for making nanocrystallites, commonly denoted quantumdots (QDs), is based on spray pyrolysis. Unlike the process used heretofore, the improved process is amenable to mass production of either passivated or non-passivated QDs, with computer control to ensure near uniformity of size.

Quantum-dot infrared photodetectors (QDIPs) are positioned to become an important technology in the field of infrared (IR) detection, particularly for high-temperature, low-cost, high-yield detector arrays required for military applications. High-operating temperature (>=150 K) photodetectors reduce the cost of IR imaging systems by enabling cryogenic dewars and Stirling cooling systems to be replaced by thermo-electric coolers. QDIPs are well-suited for detecting mid-IR light at elevated temperatures, an application that could prove to be the next commercial market for quantumdots. While quantumdot epitaxial growth and intraband absorption of IR radiation are well established, quantumdot non-uniformity remains as a significant challenge. Nonetheless, state-of-the-art mid-IR detection at 150 K has been demonstrated using 70-layer InAs/GaAs QDIPs, and QDIP focal plane arrays are approaching performance comparable to HgCdTe at 77 K. By addressing critical challenges inherent to epitaxial QD material systems (e.g., controlling dopant incorporation), exploring alternative QD systems (e.g., colloidal QDs), and using bandgap engineering to reduce dark current and enhance multi-spectral detection (e.g. resonant tunneling QDIPs), the performance and applicability of QDIPs will continue to improve.

We demonstrate a straightforward way to obtain single well-isolated quantumdots emitting in the visible part of the spectrum and characterize the optical emission from single quantumdots using this method. Self-assembled InP quantumdots are grown using gas-source molecular-beam epitaxy over a wide range of InP deposition rates, using an ultralow growth rate of about 0.01 atomic monolayers/s, a quantum-dot density of 1 dot/{mu}m{sup 2} is realized. The resulting isolated InP quantumdots embedded in an InGaP matrix are individually characterized without the need for lithographical patterning and masks on the substrate. Such low-density quantumdots show excitonic emission at around 670 nm with a linewidth limited by instrument resolution. This system is applicable as a single-photon source for applications such as quantum cryptography.

Semiconductor nanoparticles (quantumdots) were studied in the context of liquid scintillator development for upcoming neutrino experiments. The unique optical and chemical properties of quantumdots are particularly promising for the use in neutrinoless double-beta decay experiments. Liquid scintillators for large scale neutrino detectors have to meet specific requirements which are reviewed, highlighting the peculiarities of quantum-dot-doping. In this paper, we report results on laboratory-scale measurements of the attenuation length and the fluorescence properties of three commercial quantumdot samples. The results include absorbance and emission stability measurements, improvement in transparency due to filtering of the quantumdot samples, precipitation tests to isolate the quantumdots from solution and energy transfer studies with quantumdots and the fluorophore PPO. PMID:25392711

Full counting statistics of electron transport is a powerful diagnostic tool for probing the nature of quantum transport beyond what is obtainable from the average current or conductance measurement alone. In particular, the non-Markovian dynamics of quantumdot molecule plays an important role in the nonequilibrium electron tunneling processes. It is thus necessary to understand the non-Markovian full counting statistics in a quantumdot molecule. Here we study the non-Markovian full counting statistics in two typical quantumdot molecules, namely, serially coupled and side-coupled double quantumdots with high quantum coherence in a certain parameter regime. We demonstrate that the non-Markovian effect manifests itself through the quantum coherence of the quantumdot molecule system, and has a significant impact on the full counting statistics in the high quantum-coherent quantumdot molecule system, which depends on the coupling of the quantumdot molecule system with the source and drain electrodes. The results indicated that the influence of the non-Markovian effect on the full counting statistics of electron transport, which should be considered in a high quantum-coherent quantumdot molecule system, can provide a better understanding of electron transport through quantumdot molecules. PMID:25752245

We investigate theoretically the balance of the static magnetic and the dynamical photon forces in the electron transport through a quantumdot in a photon cavity with a single photon mode. The quantumdot system is connected to external leads and the total system is exposed to a static perpendicular magnetic field. We explore the transport characteristics through the system by tuning the ratio, [Formula: see text], between the photon energy, [Formula: see text], and the cyclotron energy, [Formula: see text]. Enhancement in the electron transport with increasing electron-photon coupling is observed when [Formula: see text]. In this case the photon field dominates and stretches the electron charge distribution in the quantumdot, extending it towards the contact area for the leads. Suppression in the electron transport is found when [Formula: see text], as the external magnetic field causes circular confinement of the charge density around the dot. PMID:27420809

We propose a heralded quantum repeater protocol based on the general interface between the circularly polarized photon and the quantumdot embedded in a double-sided optical microcavity. Our effective time-bin encoding on photons results in the deterministic faithful entanglement distribution with one optical fiber for the transmission of each photon in our protocol, not two or more. Our efficient parity-check detector implemented with only one input-output process of a single photon as a result of cavity quantum electrodynamics makes the entanglement channel extension and entanglement purification in quantum repeater far more efficient than others, and it has the potential application in fault-tolerant quantum computation as well. Meanwhile, the deviation from a collective-noise channel leads to some phase-flip errors on the nonlocal electron spins shared by the parties and these errors can be depressed by our simplified entanglement purification process. Finally, we discuss the performance of our proposal, concluding that it is feasible with current technology.

A strongly capacitively-coupled parallel double quantumdot was studied as a single-electron switch. The double dot was fabricated in a two-dimensional electron gas (2DEG) in a GaAs/AlGaAs heterostructure. An electrically-floating coupling gate increased capacitive-coupling between the dots, while an etched trench prevented tunnel-coupling between them. Split Coulomb blockade peaks were observed in each dot, and the Coulomb blockade conductance of the double dot formed a hexagonal pattern characteristic of coupled dots. A fractional peak splitting f = 0.34 was measured, which corresponds to a fractional capacitive-coupling alpha ≡ CINT/CSigma = 0.20. This is an order of magnitude larger than reported for similar lateral quantumdots, and shows that the coupling gate works. The strong capacitive-coupling in our device allowed the charge state of one dot to strongly influence the conductance of the other dot and enabled it to work as a single-electron switch. By moving in a combination of gate voltages, electrons are induced in one dot (the "trigger" dot) only. In response to the change in the charge state, the conductance of the other dot (the "switched" dot) is turned on and off. The abruptness of the conductance switching in gate voltage (the switching lineshape) is determined by how well charge is quantized on the trigger dot, and was found to follow tanh and arctan forms for (respectively) good and poor charge quantization in the trigger dot. A few-electron tunnel-coupled series double dot was studied for possible application to quantum computing. The device was fabricated in a square-well 2DEG in a GaAs/AlGaAs heterostructure. The dots were emptied of electrons in order to define the absolute number of electrons in the dot. Finite bias Coulomb blockade measurements on each dot showed that the last Coulomb blockade diamonds did not close and thus that both dots could be emptied. A three-dimensional conductance measurement of one dot in the one sidegate and the

We investigate the possibility of achieving scalable photonic quantum computing by the giant optical circular birefringence induced by a quantum-dot spin in a double-sided optical microcavity as a result of cavity quantum electrodynamics. We construct a deterministic controlled-not gate on two photonic qubits by two single-photon input-output processes and the readout on an electron-medium spin confined in an optical resonant microcavity. This idea could be applied to multi-qubit gates on photonic qubits and we give the quantum circuit for a three-photon Toffoli gate. High fidelities and high efficiencies could be achieved when the side leakage to the cavity loss rate is low. It is worth pointing out that our devices work in both the strong and the weak coupling regimes. PMID:23938640

A solid state model for quantum computation has potential advantages in terms of the ease of fabrication, characterization, and integration. The fundamental requirements for a quantum computer involve the realization of basic processing units (qubits), and a scheme for controlled switching and coupling among the qubits, which enables one to perform controlled operations on qubits. We propose a model for quantum computation based on optically coupled quantumdot arrays, which is computationally similar to the atomic model proposed by Cirac and Zoller. In this model, individual qubits are comprised of two coupled quantumdots, and an array of these basic units is placed in an optical cavity. Switching among the states of the individual units is done by controlled laser pulses via near field interaction using the NSOM technology. Controlled rotations involving two or more qubits are performed via common cavity mode photon. We have calculated critical times, including the spontaneous emission and switching times, and show that they are comparable to the best times projected for other proposed models of quantum computation. We have also shown the feasibility of accessing individual quantumdots using the NSOM technology by calculating the photon density at the tip, and estimating the power necessary to perform the basic controlled operations. We are currently in the process of estimating the decoherence times for this system; however, we have formulated initial arguments which seem to indicate that the decoherence times will be comparable, if not longer, than many other proposed models.

We perform high-resolution reflection spectroscopy of a quantumdot resonantly coupled to a pillar microcavity. We show the change in reflectivity as the quantumdot is tuned through the cavity resonance and measure the quantum-dot-induced phase shift using an ultrastable interferometer. The macroscopic phase shift we measure could be extended to the study of charged quantumdot pillar microcavity systems, where it could be exploited to realize a high-efficiency spin photon interface for hybrid quantum information schemes.

We present a reconfigurable metal-oxide-semiconductor multi-gate transistor that can host a quadruple quantumdot in silicon. The device consists of an industrial quadruple-gate silicon nanowire field-effect transistor. Exploiting the corner effect, we study the versatility of the structure in the single quantumdot and the serial double quantumdot regimes and extract the relevant capacitance parameters. We address the fabrication variability of the quadruple-gate approach which, paired with improved silicon fabrication techniques, makes the corner state quantumdot approach a promising candidate for a scalable quantum information architecture.

Using electrochemical charge injection, the fluorescence lifetimes of negatively charged core/shell CdTe/CdSe QDs are measured as a function of core size and shell thickness. It is found that the ensemble negative trion lifetimes reach a maximum (∼4.5 ns) for an intermediate shell thickness. This leads to the smallest particles (∼4.5 nm) with the brightest trion to date. Single dot measurements show that the negative charge suppresses blinking and that the trion can be as bright as the exciton at room temperature. In contrast, the biexciton lifetimes remain short and exhibit only a monotonous increase with shell thickness, showing no correlation with the negative trion decays. The suppression of the Auger process in small negatively charged CdTe/CdSe quantumdots is unprecedented and a significant departure from prior results with ultrathick CdSe/CdS core/shell or dot-in-rod structures. The proposed reason for the optimum shell thickness is that the electron-hole overlap is restricted to the CdTe core while the electron is tuned to have zero kinetic energy in the core for that optimum shell thickness. The different trend of the biexciton lifetime is not explained but tentatively attributed to shorter-lived positive trions at smaller sizes. These results improve our understanding of multiexciton recombination in colloidal quantumdots and may lead to the design of bright charged QDs for more efficient light-emitting devices. PMID:24350673

We present a simple measure of the conductance fluctuations in open ballistic chaotic quantumdots, extending the number of maxima method originally proposed for the statistical analysis of compound nuclear reactions. The average number of extreme points (maxima and minima) in the dimensionless conductance T as a function of an arbitrary external parameter Z is directly related to the autocorrelation function of T(Z). The parameter Z can be associated with an applied gate voltage causing shape deformation in quantumdot, an external magnetic field, the Fermi energy, etc. The average density of maxima is found to be ⟨ρZ⟩=αZ/Zc, where αZ is a universal constant and Zc is the conductance autocorrelation length, which is system specific. The analysis of ⟨ρZ⟩ does not require large statistic samples, providing a quite amenable way to access information about parametric correlations, such as Zc.

In this study, novel carbohydrate-based nanoconjugates combining chemically modified chitosan with semiconductor quantumdots (QDs) were designed and synthesised via single-step aqueous route at room temperature. Glycol chitosan (G-CHI) was used as the capping ligand aiming to improve the water solubility of the nanoconjugates to produce stable and biocompatible colloidal systems. UV-visible (UV-vis) spectroscopy, photoluminescence (PL) spectroscopy, and Fourier transform infrared (FTIR) spectroscopy were used to characterise the synthesis and the relative stability of biopolymer-capped semiconductor nanocrystals. The results clearly demonstrated that the glycol chitosan derivative was remarkably effective at nucleating and stabilising semiconductor CdS quantumdots in aqueous suspensions under acidic, neutral, and alkaline media with an average size of approximately 2.5 nm and a fluorescent activity in the visible range of the spectra.

We report on the application of the bio-conjugated quantumdots (QDs) for a "sandwich" enzyme-linked immunosorbent assay (ELISA) cancer testing technique. Quantumdot ELISA detection of the cancer PSA antigen at concentrations as low as 0.01 ng/ml which is ˜50 times lower than the classic "sandwich" ELISA was demonstrated. Scanning photoluminescence (PL) spectroscopy was performed on dried ELISA wells and the results compared with the same QD samples dried on a solid substrate. We confirmed a "blue" up to 37 nm PL spectral shift in a case of QDs conjugated to PSA antibodies. Increasing of the "blue" spectral shift was observed at lower PSA antigen concentrations. The results can be used to improve sensitivity of "sandwich" ELISA cancer antigen detection.

We demonstrate a novel quantumdot based probe with inherent signal amplification upon interaction with a targeted proteolytic enzyme. This probe may be useful for imaging in cancer detection and diagnosis. In this system, quantumdots (QDs) are bound to gold nanoparticles (AuNPs) via a proteolytically-degradable peptide sequence to non-radiatively suppress luminescence. A 71% reduction in luminescence was achieved with conjugation of AuNPs to QDs. Peptide cleavage results in release of AuNPs and restores radiative QD photoluminescence. Initial studies observed a 52% rise in luminescence over 47 hours of exposure to 0.2 mg/mL collagenase. These probes can be customized for targeted degradation simply by changing the sequence of the peptide linker.

The separability and Runge–Lenz-type dynamical symmetry of the internal dynamics of certain two-electron QuantumDots, found by Simonović et al. (2003), are traced back to that of the perturbed Kepler problem. A large class of axially symmetric perturbing potentials which allow for separation in parabolic coordinates can easily be found. Apart from the 2:1 anisotropic harmonic trapping potential considered in Simonović and Nazmitdinov (2013), they include a constant electric field parallel to the magnetic field (Stark effect), the ring-shaped Hartmann potential, etc. The harmonic case is studied in detail. -- Highlights: • The separability of QuantumDots is derived from that of the perturbed Kepler problem. • Harmonic perturbation with 2:1 anisotropy is separable in parabolic coordinates. • The system has a conserved Runge–Lenz type quantity.

A method, which makes it possible to obtain semiconductor particles V Almost-Equal-To 10{sup -20} cm{sup 3} in volume (quantumdots) with a concentration of up to 10{sup 11} cm{sup -2} and electrical contacts to each of them, is suggested. High variability in the electrical properties of such particles from a metal oxide (CuO or NiO) after the chemisorption of gas molecules is found.

An inverted architecture of quantumdot solar cells is demonstrated by introducing a novel ZnO method on top of the PbS CQD film. Improvements in device characteristics stem from constructive optical interference from the ZnO layer that enhances absorption in the PbS CQD layer. Outstanding diode characteristics arising from a superior PbS/ZnO junction provide a further electronic advantage. PMID:24677118

We study quantum many-body effects on the real-time evolution of the current through quantumdots. By using a non-equilibrium renormalization group approach, we provide analytic results for the relaxation dynamics into the stationary state and identify the microscopic cutoff scales that determine the transport rates. We find rich non-equilibrium physics induced by the interplay of the different energy scales. While the short-time limit is governed by universal dynamics, the long-time behavior features characteristic oscillations as well as an interplay of exponential and power-law decay.

Highly efficient and stable photoluminescence (PL) are urgently desired for graphene quantumdots (GQDs) to facilitate their prospective applications as optical materials. Here, we report the facile and straightforward synthesis of alkylated graphene quantumdots (AGQDs) via the solvothermal reaction of propagatively alkylated graphene sheets (PAGenes). In contrast to most GQDs reported so far, the synthesized AGQDs process pH-independent and ultra-bright PL with a relative quantum yield of up to 65%. Structural and chemical composition characterization demonstrated that the synthesized AGQDs are nearly oxygen-defect-free with alkyl groups decorated on edges and basal plane, which may contribute to their greatly improved pH tolerance and high quantum efficiency. The photocatalytic performance of AGQDs-P25 nanocomposites was evaluated by the degradation of Rhodamine B under visible light. The photocatalytic rate is ca. 5.9 times higher than that of pure P25, indicating that AGQDs could harness the visible spectrum of sunlight for energy conversion or environmental therapy.Highly efficient and stable photoluminescence (PL) are urgently desired for graphene quantumdots (GQDs) to facilitate their prospective applications as optical materials. Here, we report the facile and straightforward synthesis of alkylated graphene quantumdots (AGQDs) via the solvothermal reaction of propagatively alkylated graphene sheets (PAGenes). In contrast to most GQDs reported so far, the synthesized AGQDs process pH-independent and ultra-bright PL with a relative quantum yield of up to 65%. Structural and chemical composition characterization demonstrated that the synthesized AGQDs are nearly oxygen-defect-free with alkyl groups decorated on edges and basal plane, which may contribute to their greatly improved pH tolerance and high quantum efficiency. The photocatalytic performance of AGQDs-P25 nanocomposites was evaluated by the degradation of Rhodamine B under visible light. The

InAs/GaAs quantumdot infrared photodetectors with different doping levels were investigated to understand the effect of quantumdot filling on both intraband and interband optical transitions. The electron filling of self-assembled InAs quantumdots was varied by direct doping of quantumdots with different concentrations. Photoresponse in the near infrared and middle wavelength infrared spectral region was observed from samples with low quantumdot filling. Although undoped quantumdots were favored for interband transitions with the absence of a second optical excitation in the near infrared region, doped quantumdots were preferred to improve intraband transitions in the middle wavelength infrared region. As a result, partial filling of quantumdot was required, to the extent of maintaining a low dark current, to enhance the dual-band photoresponse through the confined electron states.

Using a deterministic single P donor placed with atomic precision accuracy next to a nanoscale silicon quantumdot, we present a way to analyze the energy spectrum of small quantumdots in silicon by tunnel-coupled transport measurements. The energy-level structure of the quantumdot is observed as resonance features within the transport bias triangles when the donor chemical potential is aligned with states within the quantumdot as confirmed by a numeric rate equation solver SIMON. This technique allows us to independently extract the quantumdot level structure irrespective of the density of states in the leads. Such a method is useful for the investigation of silicon quantumdots in the few-electron regime where the level structure is governed by an intricate interplay between the spin- and the valley-orbit degrees of freedom.

GaAs-based long-wavelength quantumdot lasers have long been studied for applications to optical interconnects. The zero-dimensional confinement potential of quantumdots opens possibility of novel devices. Also, the quantumdot itself shows very interesting characteristics. This dissertation describes the development of GaAs-based 1.3 μm quantumdot lasers and the research on the unique characteristics of quantumdot ensemble. InGaAs quantumdots grown using molecular beam epitaxy in submonolayer deposition have extended wavelength around 1.3 μm and well resolved energy levels that can be described by three-dimensional harmonic oscillator model assuming parabolic confining potential. Lasing transitions from various InGaAs quantumdot energy levels are obtained from edge-emitting lasers. With optimized quantumdot active region and device structure, continuous-wave, room-temperature lasing operation around 1.3 μm is achieved with very low threshold current. Lateral confinement of carriers and photons in the cavity with AlxO y using wet-oxidation technique results in low waveguide loss, which lowers the threshold further. InGaAs quantumdot lasers have almost temperature- insensitive lasing threshold below ~200 K with very low threshold current density close to transparency current density. The rapid increase of threshold current along with temperature above ~200 K is due to thermal excitation of carriers into the higher energy levels and increase of non-radiative recombination. Quasi- equilibrium model for carrier dynamics shows that the optical gain of quantumdot ensemble is strongly temperature dependent, and that the separation between quantumdot energy levels plays an important role in the temperature dependence of the device characteristics. Several predictions of the model are compared with the experimental results. Lasing operation with less temperature-sensitivity is achieved from InAs quantumdot lasers with increased level separation.

Quantum logic gates are the key elements in quantum computing. Here we investigate the possibility of achieving a scalable and compact quantum computing based on stationary electron-spin qubits, by using the giant optical circular birefringence induced by quantum-dot spins in double-sided optical microcavities as a result of cavity quantum electrodynamics. We design the compact quantum circuits for implementing universal and deterministic quantum gates for electron-spin systems, including the two-qubit CNOT gate and the three-qubit Toffoli gate. They are compact and economic, and they do not require additional electron-spin qubits. Moreover, our devices have good scalability and are attractive as they both are based on solid-state quantum systems and the qubits are stationary. They are feasible with the current experimental technology, and both high fidelity and high efficiency can be achieved when the ratio of the side leakage to the cavity decay is low. PMID:25518899

Quantum logic gates are the key elements in quantum computing. Here we investigate the possibility of achieving a scalable and compact quantum computing based on stationary electron-spin qubits, by using the giant optical circular birefringence induced by quantum-dot spins in double-sided optical microcavities as a result of cavity quantum electrodynamics. We design the compact quantum circuits for implementing universal and deterministic quantum gates for electron-spin systems, including the two-qubit CNOT gate and the three-qubit Toffoli gate. They are compact and economic, and they do not require additional electron-spin qubits. Moreover, our devices have good scalability and are attractive as they both are based on solid-state quantum systems and the qubits are stationary. They are feasible with the current experimental technology, and both high fidelity and high efficiency can be achieved when the ratio of the side leakage to the cavity decay is low. PMID:25518899

An essential parameter to evaluate the light emission properties of fluorophores is the fluorescence quantum yield, which quantify the conversion efficiency of absorbed photons to emitted photons. We detail here an alternative nonfluorescent method to determine the absolute fluorescence quantum yield of quantumdots (QDs). The method is based in the so-called Thermal Lens Spectroscopy (TLS) technique, which consists on the evaluation of refractive index gradient thermally induced in the fluorescent material by the absorption of light. Aqueous dispersion carboxyl-coated cadmium telluride (CdTe) QDs samples were used to demonstrate the Thermal Lens Spectroscopy technical procedure. PMID:25103802

A recent experiment [E. A. Chekhovich et al., Phys. Rev. Lett. 104, 066804 (2010), 10.1103/PhysRevLett.104.066804] has demonstrated that high nuclear spin polarization can be achieved in self-assembled quantumdots by exploiting an optically forbidden transition between a heavy hole and a trion state. However, a fully polarized state is not achieved as expected from a classical rate equation. Here, we theoretically investigate this problem with the help of a quantum master equation and we demonstrate that a fully polarized state cannot be achieved due to formation of a nuclear dark state. Moreover, we show that the maximal degree of polarization depends on structural properties of the quantumdot.

Tailored optical output, such as color purity and efficient optical intensity, are critical considerations for displays, particularly in mobile applications. To this end, we demonstrate a replica molded photonic crystal structure with embedded quantumdots. Electrohydrodynamic jet printing is used to control the position of the quantumdots within the device structure. This results in significantly less waste of the quantumdot material than application through drop-casting or spin coating. In addition, the targeted placement of the quantumdots minimizes any emission outside of the resonant enhancement field, which enables an 8× output enhancement and highly polarized emission from the photonic crystal structure.

In this paper we report two different methodologies for labeling ligand-gated receptors. The first of these builds upon our earlier work with serotonin conjugated quantumdots and our studies with pegilated quantumdots to reduce non specific binding. In this approach a pegilated derivative of muscimol was synthesized and attached via an amide linkage to quantumdots coated in an amphiphillic polymer derivative of poly acrylamide. These conjugates were used to image the GABA C receptor in oocytes. An alternative approach was used to image tissue sections to study nicotinic acetylcholine receptors in the neuro muscular junction with biotinylated Bungerotoxin and streptavidin coated quantumdots.

The characteristics of quantumdot solar cells (QDSCs) based on type II QDs are investigated theoretically. Based on a drift-diffusion model, we obtained a much higher open circuit voltage (V{sub oc}) as well as conversion efficiency in a type II QDSC, compared to type I QDSCs. The improved V{sub oc} and efficiency are mainly attributed to the much longer Auger recombination lifetime in type II QDs. Moreover, the influence of the carrier lifetime on devices' performance is discussed and clarified. In addition, an explicit criterion to determine the role of quantumdots in solar cells is put forward.

Electron spins in semiconductor quantumdots are good candidates of quantum bits for quantum information processing. Basic operations of the qubit have been realized in recent years: initialization, manipulation of single spins, two qubit entanglement operations, and readout. Now it becomes crucial to demonstrate scalability of this architecture by conducting spin operations on a scaled up system. Here, we demonstrate single-electron spin resonance in a quadruple quantumdot. A few-electron quadruple quantumdot is formed within a magnetic field gradient created by a micro-magnet. We oscillate the wave functions of the electrons in the quantumdots by applying microwave voltages and this induces electron spin resonance. The resonance energies of the four quantumdots are slightly different because of the stray field created by the micro-magnet and therefore frequency-resolved addressable control of each electron spin resonance is possible. PMID:27550534

Quantumdots in semiconductor heterostructures provide one of the most flexible platforms for the study of quantum phenomena at the nanoscale. The surging interest in using quantumdots for quantum computation is forcing researchers to rethink fabrication and operation methods, to obtain highly tunable dots in spin-free host materials, such as silicon. Borselli and colleagues report in Nanotechnology the fabrication of a novel Si/SiGe double quantumdot device, which combines an ultra-low disorder Si/SiGe accumulation-mode heterostructure with a stack of overlapping control gates, ensuring tight confining potentials and exquisite tunability. This work signals the technological maturity of silicon quantumdots, and their readiness to be applied to challenging projects in quantum information science.

Electron spins in semiconductor quantumdots are good candidates of quantum bits for quantum information processing. Basic operations of the qubit have been realized in recent years: initialization, manipulation of single spins, two qubit entanglement operations, and readout. Now it becomes crucial to demonstrate scalability of this architecture by conducting spin operations on a scaled up system. Here, we demonstrate single-electron spin resonance in a quadruple quantumdot. A few-electron quadruple quantumdot is formed within a magnetic field gradient created by a micro-magnet. We oscillate the wave functions of the electrons in the quantumdots by applying microwave voltages and this induces electron spin resonance. The resonance energies of the four quantumdots are slightly different because of the stray field created by the micro-magnet and therefore frequency-resolved addressable control of each electron spin resonance is possible. PMID:27550534

Highly efficient and stable photoluminescence (PL) are urgently desired for graphene quantumdots (GQDs) to facilitate their prospective applications as optical materials. Here, we report the facile and straightforward synthesis of alkylated graphene quantumdots (AGQDs) via the solvothermal reaction of propagatively alkylated graphene sheets (PAGenes). In contrast to most GQDs reported so far, the synthesized AGQDs process pH-independent and ultra-bright PL with a relative quantum yield of up to 65%. Structural and chemical composition characterization demonstrated that the synthesized AGQDs are nearly oxygen-defect-free with alkyl groups decorated on edges and basal plane, which may contribute to their greatly improved pH tolerance and high quantum efficiency. The photocatalytic performance of AGQDs-P25 nanocomposites was evaluated by the degradation of Rhodamine B under visible light. The photocatalytic rate is ca. 5.9 times higher than that of pure P25, indicating that AGQDs could harness the visible spectrum of sunlight for energy conversion or environmental therapy. PMID:25192187

The surface coating of quantumdots has been characterised using Z-stem. Quantumdots have been pegylated to increase stability in aqueous solution. The fluorescence intensity of the quantumdots was modulated pegylation. PEG was coupled using different ratios of EDC, PEG and NHS. Optimum coupling conditions were found to occur when 2000 equivalents of PEG were reacted with 1 equivalent of dot in the presence of 1500 equivalents of NHS and EDC. Angiotensin II was also conjugated to quantumdots and these conjugates were shown to be biologically active. Quantumdots have also been surface functionalised with other peptides such as NGR with subsequent demonstration of cell surface binding and can be characterized by flow cytometry.

We show how single quantumdots, each hosting a singlet-triplet qubit, can be placed in arrays to build a spin quantum cellular automaton. A fast (˜10 ns) deterministic coherent singlet-triplet filtering, as opposed to current incoherent tunneling/slow-adiabatic based quantum gates (operation time ˜300 ns), can be employed to produce a two-qubit gate through capacitive (electrostatic) couplings that can operate over significant distances. This is the coherent version of the widely discussed charge and nano-magnet cellular automata, and would increase speed, reduce dissipation, and perform quantum computation while interfacing smoothly with its classical counterpart. This combines the best of two worlds—the coherence of spin pairs known from quantum technologies, and the strength and range of electrostatic couplings from the charge-based classical cellular automata. Significantly our system has zero electric dipole moment during the whole operation process, thereby increasing its charge dephasing time.

We studied the chiroptical properties of colloidal solution of CdSe and CdSe/ZnS quantumdots (QDs) with a cubic lattice structure which were initially prepared without use of any chiral molecules and coated with achiral ligands. We demonstrate circular dichroism (CD) activity around first and second excitonic transition of these CdSe based nanocrystals. We consider that this chiroptical activity is caused by imbalance in racemic mixtures of QDs between the left and right handed nanoparticles, which appears as a result of the formation of various defects or incorporation of impurities into crystallographic structure during their synthesis. We demonstrate that optical activity of colloidal solution of CdSe QDs with achiral ligands weakly depends on the QDs size and number of ZnS monolayers, but does not depend on the nature of achiral ligands or polarity of the solution. PMID:26832599

We present a general approach to calculating the entanglement of formation for superpositions of two-mode coherent states, placed equidistantly on a circle in phase space. We show that in the particular case of rotationally invariant circular states the Schmidt decomposition of two modes, and therefore the value of their entanglement, are given by analytical expressions. We analyze the dependence of the entanglement on the radius of the circle and number of components in the superposition. We also show that the set of rotationally invariant circular states creates an orthonormal basis in the state space of the harmonic oscillator, and this basis is advantageous for representation of other circular states of light.

An experiment is conducted for obtaining quantumdots for physical or materials chemistry. This experiment serves to both reinforce the basic concept of quantum confinement and providing a useful bridge between the molecular and solid-state world.

Nanocrystal quantumdots are attractive materials for applications as nanoscale light sources. One impediment to these applications is fluctuations of single-dot emission intensity, known as blinking. Recent progress in colloidal synthesis has produced nonblinking nanocrystals; however, the physics underlying blinking suppression remains unclear. Here we find that ultra-thick-shell CdSe/CdS nanocrystals can exhibit pronounced fluctuations in the emission lifetimes (lifetime blinking), despite stable nonblinking emission intensity. We demonstrate that lifetime variations are due to switching between the neutral and negatively charged state of the nanocrystal. Negative charging results in faster radiative decay but does not appreciably change the overall emission intensity because of suppressed nonradiative Auger recombination for negative trions. The Auger process involving excitation of a hole (positive trion pathway) remains efficient and is responsible for charging with excess electrons, which occurs via Auger-assisted ionization of biexcitons accompanied by ejection of holes. PMID:22713750

We show that the application of a vertical electric field to the Coulomb interacting system in stacked quantumdots leads to a 90° in-plane switching of charge probability distribution in contrast to a single dot, where no such switching exists. Results are obtained using path integral quantum Monte Carlo with realistic dot geometry, alloy composition, and piezo-electric potential profiles. The origin of the switching lies in the strain interactions between the stacked dots hence the need for more than one layer of dots. The lateral polarization and electric field dependence of the radiative lifetimes of the excitonic switch are also discussed.

A new conception of nano-laser is proposed in which depending on the size of nano-clusters (silicon quantumdots (QD)), the pumping level of laser can be tuned by the quantum confinement (QC) effect, and the population inversion can be formed between the valence band and the localized states in gap produced from the surface bonds of nano-clusters. Here we report the experimental demonstration of nano-laser on silicon quantumdots fabricated by nanosecond pulse laser. The peaks of stimulated emission are observed at 605 nm and 693 nm. Through the micro-cavity of nano-laser, a full width at half maximum of the peak at 693 nm can reach to 0.5 nm. The theoretical model and the experimental results indicate that it is a necessary condition for setting up nano-laser that the smaller size of QD (d < 3 nm) can make the localized states into band gap. The emission energy of nano-laser will be limited in the range of 1.7-2.3 eV generally due to the position of the localized states in gap, which is in good agreement between the experiments and the theory.

Some quantumdot samples show a long-time (power-law) behavior of their luminescence intensity decay. This effect has been recently explained as being due to a cooperation of many tunneling channels transferring electrons from small quantumdots with triplet exciton to quantumdots at which the electrons can recombine with the holes in the valence band states. In this work we show that the long-time character of the sample luminescence decay can also be caused by an intrinsic property of a single dot, namely, by a non-adiabatic effect of the electron occupation up-conversion caused by the electron-phonon multiple scattering mechanism.

Coupling a semiconductor quantumdot qubit to a superconducting resonator broadens the possibilities for interqubit communication and potentially allows integration of quantumdots with other qubit systems. The major technological hurdle that must be overcome is reaching the strong coupling limit, where the coupling frequency between the resonator and the qubit is larger than both the qubit decoherence rate and the photon loss rate of the resonator. In this work, we examine optimization of the quantumdot device design. Using the Thomas-Fermi approximation in conjunction with a metallic dot capacitive model, we focus on improving the capacitive coupling between a resonator gate and a quantumdot while decreasing the cross-coupling to nearby dots. Through these simulations, we find that the optimization follows an intuitive geometric relation. This work was supported in part by ARO (W911NF-12-0607), NSF (PHY-1104660), and ONR (N00014-15-1-0029).

We study quantum Hall interferometers in which the interference loop encircles a quantum anti-dot. We base our study on thermodynamic considerations, which we believe reflect the essential aspects of interference transport phenomena. We find that similar to the more conventional Fabry-Perot quantum Hall interferometers, in which the interference loop forms a quantumdot, the anti-dot interferometer is affected by the electro-static Coulomb interaction between the edge modes defining the loop. We show that in the Aharonov-Bohm regime, in which effects of fractional statistics should be visible, is easier to access in interferometers based on anti-dots than in those based on dots. We discuss the relevance of our results to recent measurements on anti-dots interferometers.

We demonstrate control of energy states in epitaxially-grown quantumdot structures formed by stacked submonolayer InAs depositions via engineering of the internal bandstructure of the dots. Transmission electron microscopy of the stacked sub-monolayer regions shows compositional inhomogeneity, indicative of the presence of quantumdots. The quantumdot ground state is manipulated not only by the number of deposited InAs layers, but also by control of the thickness and material composition of the spacing layers between submonolayer InAs depositions. In this manner, we demonstrate the ability to shift the quantumdot ground state energy at 77 K from 1.38 eV to 1.88 eV. The results presented offer a potential avenue towards enhanced control of dot energies for a variety of optoelectronic applications.

We report the first demonstration of a hybrid silicon quantumdot (QD) laser, evanescently coupled to a silicon waveguide. InAs/GaAs QD laser structures with thin AlGaAs lower cladding layers were transferred by direct wafer bonding onto silicon waveguides defining cavities with adiabatic taper structures and distributed Bragg reflectors. The laser operates at temperatures up to 115 °C under pulsed current conditions, with a characteristic temperature T 0 of 303 K near room temperature. Furthermore, by reducing the width of the GaAs/AlGaAs mesa down to 8 µm, continuous-wave operation is realized at 25 °C.

Graphene, which exhibits a massless Dirac-like spectrum for its electrons, has shown impressive properties for nano-electronics applications including a high mobility and a width dependent bandgap. We will report the preliminary report on the transport property of the suspended graphene nano-ribbon(GNR) quantumdot device down to dilution refrigerator temperature. This GNR QD device was fabricated to realize an ideal probe to investigate Kondo physics--a characteristic phenomenon in the physics of strongly correlated electrons. We gratefully acknowledge the financial support by the National Research Foundation of Korea through the Creative Research Initiatives.

We propose the interaction of two electrons in a triple quantumdot as a minimal system to control long-range superexchange transitions. These are probed by transport spectroscopy. Narrow resonances appear indicating the transfer of charge from one side of the sample to the other with the central one being occupied only virtually. We predict that two different intermediate states establish the two arms of a one-dimensional interferometer. We find configurations where destructive interference of the two superexchange trajectories totally blocks the current through the system. We emphasize the role of spin correlations giving rise to lifetime-enhanced resonances.

In a mesoscopic system, under zero bias voltage, a finite charge is transferred by quantum adiabatic pumping by adiabatically and periodically changing two or more control parameters. We obtained expressions for the pumped charge for a ring of three quantumdots (QDs) by choosing the magnetic flux penetrating the ring as one of the control parameters. We found that the pumped charge shows a steplike behavior with respect to the variance of the flux. The value of the step heights is not universal but depends on the trajectory of the control parameters. We discuss the physical origin of this behavior on the basis of the Fano resonant condition of the ring.

Green emission ZnO quantumdots were synthesized by an ultrasonic sol-gel method. The ZnO quantumdots were synthesized in various ultrasonic temperature and time. Photoluminescence properties of these ZnO quantumdots were measured. Time-resolved photoluminescence decay spectra were also taken to discover the change of defects amount during the reaction. Both ultrasonic temperature and time could affect the type and amount of defects in ZnO quantumdots. Total defects of ZnO quantumdots decreased with the increasing of ultrasonic temperature and time. The dangling bonds defects disappeared faster than the optical defects. Types of optical defects first changed from oxygen interstitial defects to oxygen vacancy and zinc interstitial defects. Then transformed back to oxygen interstitial defects again. The sizes of ZnO quantumdots would be controlled by both ultrasonic temperature and time as well. That is, with the increasing of ultrasonic temperature and time, the sizes of ZnO quantumdots first decreased then increased. Moreover, concentrated raw materials solution brought larger sizes and more optical defects of ZnO quantumdots. PMID:26611814

Quantumdots enable strong carrier confinement and exhibit a delta-function like density of states, offering significant improvements to laser performance and high-temperature stability when used as a gain medium. However, quantumdot lasers have been limited to photonic cavities that are diffraction-limited and further miniaturization to meet the demands of nanophotonic-electronic integration applications is challenging based on existing designs. Here we introduce the first quantumdot-based plasmonic laser to reduce the cross-sectional area of nanowire quantumdot lasers below the cutoff limit of photonic modes while maintaining the length in the order of the lasing wavelength. Metal organic chemical vapor deposition grown GaAs-AlGaAs core-shell nanowires containing InGaAs quantumdot stacks are placed directly on a silver film, and lasing was observed from single nanowires originating from the InGaAs quantumdot emission into the low-loss higher order plasmonic mode. Lasing threshold pump fluences as low as ∼120 μJ/cm(2) was observed at 7 K, and lasing was observed up to 125 K. Temperature stability from the quantumdot gain, leading to a high characteristic temperature was demonstrated. These results indicate that high-performance, miniaturized quantumdot lasers can be realized with plasmonics. PMID:27030886

We study the optical properties of highly elongated, highly flattened quantumdot structures, also referred to as quantum dashes, characterized by the presence of two trapping centers located along the structure. Such a system can exhibit some of the properties characteristic for double quantumdots. We show that sub- and super-radiant states can form for certain quantum dash geometries, which is manifested by a pronounced transfer of intensity between spectral lines, accompanied by the appearance of strong electron-hole correlations. We also compare exciton absorption spectra and polarization properties of a system with a single and double trapping center and show how the geometry of multiple trapping centers influences the optical properties of the system. We show that for a broad range of trapping geometries the relative absorption intensity of the ground state is larger than that of the lowest excited states, contrary to the quantum dash systems characterized by a single trapping center. Thus, optical properties of these structures are determined by fine details of their morphology.

We use multi-population rate equations model to study feedback oscillations in the quantumdot laser. This model takes into account all peculiar characteristics in the quantumdots such as inhomogeneous broadening of the gain spectrum, the presence of the excited states on the quantumdot and the non-confined states due to the presence of wetting layer and the barrier. The contribution of quantumdot groups, which cannot follow by other models, is simulated. The results obtained from this model show the feedback oscillations, the periodic oscillations which evolves to chaos at higher injection current of higher feedback levels. The frequency fluctuation is attributed mainly to wetting layer with a considerable contribution from excited states. The simulation shows that is must be not using simple rate equation models to express quantumdots working at excited state transition.

Colloidal semiconductor quantumdots are fluorescent nanocrystals exhibiting exceptional optical properties, but their emission intensity strongly depends on their charging state and local environment. This leads to blinking at the single-particle level or even complete fluorescence quenching, and limits the applications of quantumdots as fluorescent particles. Here, we show that a single quantumdot encapsulated in a silica shell coated with a continuous gold nanoshell provides a system with a stable and Poissonian emission at room temperature that is preserved regardless of drastic changes in the local environment. This novel hybrid quantumdot/silica/gold structure behaves as a plasmonic resonator with a strong Purcell factor, in very good agreement with simulations. The gold nanoshell also acts as a shield that protects the quantumdot fluorescence and enhances its resistance to high-power photoexcitation or high-energy electron beams. This plasmonic fluorescent resonator opens the way to a new family of plasmonic nanoemitters with robust optical properties.

We propose a quantumdot qubit architecture that has an attractive combination of speed and fabrication simplicity. It consists of a double quantumdot with one electron in one dot and two electrons in the other. The qubit itself is a set of two states with total spin quantum numbers S2=3/4 (S=1/2) and Sz=-1/2, with the two different states being singlet and triplet in the doubly occupied dot. Gate operations can be implemented electrically and the qubit is highly tunable, enabling fast implementation of one- and two-qubit gates in a simpler geometry and with fewer operations than in other proposed quantumdot qubit architectures with fast operations. Moreover, the system has potentially long decoherence times. These are all extremely attractive properties for use in quantum information processing devices.

Luminescent semiconductor nanocrystals or quantumdots (QDs) are a recently developed class of nanomaterial whose unique photophysical properties are helping to create a new generation of robust fluorescent biosensors. QD properties of interest for biosensing include high quantum yields, broad absorption spectra coupled to narrow size-tunable photoluminescent emissions and exceptional resistance to both photobleaching and chemical degradation. In this review, we examine the progress in adapting QDs for several predominantly in vitro biosensing applications including use in immunoassays, as generalized probes, in nucleic acid detection and fluorescence resonance energy transfer (FRET) - based sensing. We also describe several important considerations when working with QDs mainly centered on the choice of material(s) and appropriate strategies for attaching biomolecules to the QDs.

Laterally localized electronic states are identified on a single layer of graphene on ruthenium by low temperature scanning tunneling spectroscopy (STS). The individual states are separated by 3 nm and comprise regions of about 90 carbon atoms. This constitutes a highly regular quantumdot-array with molecular precision. It is evidenced by quantum well resonances (QWRs) with energies that relate to the corrugation of the graphene layer. The dI/dV conductance spectra are modeled by a layer height dependent potential-well with a delta-function potential that describes the barrier for electron penetration into graphene. The resulting QWRs are strongest and lowest in energy on the isolated 'hill' regions with a diameter of 2 nm, where the graphene is decoupled from the surface.

In this work, the electronic structures of quantumdots (QDs) of nine direct band gap semiconductor materials belonging to the group II-VI and III-V families are investigated, within the empirical tight-binding framework, in the effective bond orbital model. This methodology is shown to accurately describe these systems, yielding, at the same time, qualitative insights into their electronic properties. Various features of the bulk band structure such as band-gaps, band curvature, and band widths around symmetry points affect the quantum confinement of electrons and holes. These effects are identified and quantified. A comparison with experimental data yields good agreement with the calculations. These theoretical results would help quantify the optical response of QDs of these materials and provide useful input for applications.

We study spectral properties of electron tunneling in double quantumdots (DQDs) (and double quantum wells (DQWs)) and their relation to the geometry. In particular we compare the tunneling in DQW with chaotic and regular geometry, taking into account recent evidence about regularization of the tunneling rate when the QW geometry is chaotic. Our calculations do not support this assumption. We confirm high influence of the QW geometry boundaries on the rate fluctuation along the spectrum. The factors of the effective mass anisotropy and violation of the symmetry of DQD and DQW are also considered. Generally, we found that the small violation of the symmetry drastically affects tunneling. This work is supported by the NSF (HRD-0833184) and NASA (NNX09AV07A).

We present analytical and numerical results that demonstrate the presence of the Braess paradox in chaotic quantumdots. The paradox that we identify, originally perceived in classical networks, shows that the addition of more capacity to the network can suppress the current flow in the universal regime. We investigate the weak localization term, showing that it presents the paradox encoded in a saturation minimum of the conductance, under the presence of hyperflow in the external leads. In addition, we demonstrate that the weak localization suffers a transition signal depending on the overcapacity lead and presents an echo on the magnetic crossover before going to zero due to the full time-reversal symmetry breaking. We also show that the quantum interference contribution can dominate the Ohm term in the presence of constrictions and that the corresponding Fano factor engenders an anomalous behavior. PMID:25375575

We present analytical and numerical results that demonstrate the presence of the Braess paradox in chaotic quantumdots. The paradox that we identify, originally perceived in classical networks, shows that the addition of more capacity to the network can suppress the current flow in the universal regime. We investigate the weak localization term, showing that it presents the paradox encoded in a saturation minimum of the conductance, under the presence of hyperflow in the external leads. In addition, we demonstrate that the weak localization suffers a transition signal depending on the overcapacity lead and presents an echo on the magnetic crossover before going to zero due to the full time-reversal symmetry breaking. We also show that the quantum interference contribution can dominate the Ohm term in the presence of constrictions and that the corresponding Fano factor engenders an anomalous behavior.

Semiconductor nanoparticles (or quantumdots, QDs) exhibit unique optical and electronic properties such as size-controlled fluorescence, high quantum yields, and stability against photobleaching. These properties allow QDs to be used as optical labels for multiplexed imaging and in drug delivery detection systems. Luminescent silicon QDs and surface-modified silicon QDs have also been developed as potential minimally toxic fluorescent probes for bioapplications. Silicon, a well-known power electronic semiconductor material, is considered an extremely biocompatible material, in particular with respect to blood. This review article summarizes existing knowledge related to and recent research progress made in the methods for synthesizing silicon QDs, as well as their optical properties and surface-modification processes. In addition, drug delivery systems and in vitro and in vivo imaging applications that use silicon QDs are also discussed. PMID:23949967

Semiconductor quantumdots (QDs) are nanometre-scale crystals, which have unique photophysical properties, such as size-dependent optical properties, high fluorescence quantum yields, and excellent stability against photobleaching. These properties enable QDs as the promising optical labels for the biological applications, such as multiplexed analysis of immunocomplexes or DNA hybridization processes, cell sorting and tracing, in vivo imaging and diagnostics in biomedicine. Meanwhile, QDs can be used as labels for the electrochemical detection of DNA or proteins. This article reviews the synthesis and toxicity of QDs and their optical and electrochemical bioanalytical applications. Especially the application of QDs in biomedicine such as delivering, cell targeting and imaging for cancer research, and in vivo photodynamic therapy (PDT) of cancer are briefly discussed. PMID:22247690

Semiconductor quantumdots (QDs) have been drawing great attention recently as a material for solar energy conversion due to their versatile optical and electrical properties. The QD-sensitized solar cell (QDSC) is one of the burgeoning semiconductor QD solar cells that shows promising developments for the next generation of solar cells. This article focuses on recent developments in QDSCs, including 1) the effect of quantum confinement on QDSCs, 2) the multiple exciton generation (MEG) of QDs, 3) fabrication methods of QDs, and 4) nanocrystalline photoelectrodes for solar cells. We also make suggestions for future research on QDSCs. Although the efficiency of QDSCs is still low, we think there will be major breakthroughs in developing QDSCs in the future. PMID:24191178

Luminescence upconversion nanocrystals capable of converting two low-energy photons into a single photon at a higher energy are sought-after for a variety of applications, including bioimaging and photovoltaic light harvesting. Currently available systems, based on rare-earth-doped dielectrics, are limited in both tunability and absorption cross-section. Here we present colloidal double quantumdots as an alternative nanocrystalline upconversion system, combining the stability of an inorganic crystalline structure with the spectral tunability afforded by quantum confinement. By tailoring its composition and morphology, we form a semiconducting nanostructure in which excited electrons are delocalized over the entire structure, but a double potential well is formed for holes. Upconversion occurs by excitation of an electron in the lower energy transition, followed by intraband absorption of the hole, allowing it to cross the barrier to a higher energy state. An overall conversion efficiency of 0.1% per double excitation event is achieved. PMID:23912060

We report on a photodetector in which colloidal quantumdots directly bridge nanometer-spaced electrodes. Unlike in conventional quantum-dot thin film photodetectors, charge mobility no longer plays a role in our quantum-dot junctions as charge extraction requires only two individual tunnel events. We find an efficient photoconductive gain mechanism with external quantum efficiencies of 38 electrons-per-photon in combination with response times faster than 300 ns. This compact device-architecture may open up new routes for improved photodetector performance in which efficiency and bandwidth do not go at the cost of one another. PMID:23094869

Colloidal quantumdot light-emitting devices (QD-LEDs) have generated considerable interest for applications such as thin film displays with improved color saturation and white lighting with a high color rendering index (CRI). We review the key advantages of using quantumdots (QDs) in display and lighting applications, including their color purity, solution processability, and stability. After highlighting the main developments in QD-LED technology in the past 15 years, we describe the three mechanisms for exciting QDs - optical excitation, Förster energy transfer, and direct charge injection - that have been leveraged to create QD-LEDs. We outline the challenges facing QD-LED development, such as QD charging and QD luminescence quenching in QD thin films. We describe how optical downconversion schemes have enabled researchers to overcome these challenges and develop commercial lighting products that incorporate QDs to achieve desirable color temperature and a high CRI while maintaining efficiencies comparable to inorganic white LEDs (>65 lumens per Watt). We conclude by discussing some current directions in QD research that focus on achieving higher efficiency and air-stable QD-LEDs using electrical excitation of the luminescent QDs. PMID:22110863

In this study, we introduce an integrability-based method enabling the study of semiconductor quantumdot models incorporating both the full hyperfine interaction as well as a mean-field treatment of dipole-dipole interactions in the nuclear spin bath. By performing free induction decay and spin echo simulations we characterize the combined effect of both types of interactions on the decoherence of the electron spin, for external fields ranging from low to high values. We show that for spin echo simulations the hyperfine interaction is the dominant source of decoherence at short times for low fields, and competes with the dipole-dipole interactions at longer times. On the contrary, at high fields the main source of decay is due to the dipole-dipole interactions. In the latter regime an asymmetry in the echo is observed. Furthermore, the non-decaying fraction previously observed for zero field free induction decay simulations in quantumdots with only hyperfine interactions, is destroyed for longer times by the mean-field treatment of the dipolar interactions.

We propose a novel concept for an integrable and flexible sensor capable to visualize mechanical impacts on lightweight structures by quenching the photoluminescence (PL) of CdSe quantumdots. Considering the requirements such as visibility, storage time and high optical contrast of PL quenching with low power consumption, we have investigated a symmetrical and an asymmetrical layer stack consisting of semiconductor organic N,N,N',N'-Tetrakis(3-methylphenyl)-3,3'-dimethylbenzidine (HMTPD) and CdSe quantumdots with elongated CdS shell. Time-resolved series of PL spectra from layer stacks with applied voltages of different polarity and simultaneous observation of power consumption have shown that a variety of mechanisms such as photo-induced charge separation and charge injection, cause PL quenching. However, mechanisms such as screening of external field as well as Auger-assisted charge ejection is working contrary to that. Investigations regarding the influence of illumination revealed that the positive biased asymmetrical layer stack is the preferred sensor configuration, due to a charge carrier injection at voltages of 10 V without the need of coincident illumination.

In this study, we introduce an integrability-based method enabling the study of semiconductor quantumdot models incorporating both the full hyperfine interaction as well as a mean-field treatment of dipole-dipole interactions in the nuclear spin bath. By performing free induction decay and spin echo simulations we characterize the combined effect of both types of interactions on the decoherence of the electron spin, for external fields ranging from low to high values. We show that for spin echo simulations the hyperfine interaction is the dominant source of decoherence at short times for low fields, and competes with the dipole-dipole interactions atmore » longer times. On the contrary, at high fields the main source of decay is due to the dipole-dipole interactions. In the latter regime an asymmetry in the echo is observed. Furthermore, the non-decaying fraction previously observed for zero field free induction decay simulations in quantumdots with only hyperfine interactions, is destroyed for longer times by the mean-field treatment of the dipolar interactions.« less

Stacked InAs quantumdot chains (QDCs) on InGaAs/GaAs cross-hatch pattern (CHP) templates yield a rich emission spectrum with an unusual carrier transfer characteristic compared to conventional quantumdot (QD) stacks. The photoluminescent spectra of the controlled, single QDC layer comprise multiple peaks from the orthogonal QDCs, the free-standing QDs, the CHP, the wetting layers and the GaAs substrate. When the QDC layers are stacked, employing a 10 nm GaAs spacer between adjacent QDC layers, the PL spectra are dominated by the top-most stack, indicating that the QDC layers are nominally uncoupled. Under high excitation power densities when the high-energy peaks of the top stack are saturated, however, low-energy PL peaks from the bottom stacks emerge as a result of carrier transfers across the GaAs spacers. These unique PL signatures contrast with the state-filling effects in conventional, coupled QD stacks and serve as a means to quickly assess the presence of electronic coupling in stacks of dissimilar-sized nanostructures.

It is intriguing that silicon, the central material of modern classical electronics, also has properties well suited to quantum electronics. Recent advances in Si/SiGe quantum devices have enabled the creation of high-quality silicon quantumdots, also known as artificial atoms. Motivated in part by the potential for very long spin coherence times in this material, we are pursuing the development of individual electron spin qubits in silicon quantumdots. I will discuss recent demonstrations of single-shot spin measurement in a Si/SiGe quantumdot spin qubit, and the demonstration of spin-relaxation times longer than one second in such a system. These and similar measurements depend on a knowledge of tunnel rates between quantumdots and nearby reservoirs or between pairs of quantumdots. Measurements of such rates provide an opportunity to revisit classic experiments in quantum mechanics. At the same time, the unique features of the silicon conduction band lead to novel and unexpected effects, demonstrating that Si/SiGe quantumdots provide a highly controlled experimental system in which to study ideas at the heart of quantum physics.

We report the first comprehensive experimental and theoretical study of the optical properties of single crystal phase quantumdots in InP nanowires. Crystal phase quantumdots are defined by a transition in the crystallographic lattice between zinc blende and wurtzite segments and therefore offer unprecedented potential to be controlled with atomic layer accuracy without random alloying. We show for the first time that crystal phase quantumdots are a source of pure single-photons and cascaded photon-pairs from type II transitions with excellent optical properties in terms of intensity and line width. We notice that the emission spectra consist often of two peaks close in energy, which we explain with a comprehensive theory showing that the symmetry of the system plays a crucial role for the hole levels forming hybridized orbitals. Our results state that crystal phase quantumdots have promising quantum optical properties for single photon application and quantum optics. PMID:26806321

Recent progress in the fabrication of quantumdots by molecular beam epitaxy along three directions in space is reviewed. The optical properties of different sample structures consisting of individual quantumdots, pairs of coupled dots as well as of linear arrays of dots are studied by microscopic photoluminescence spectroscopy. The high degree of control over shape, composition and position of the 7×7×7 nm3 size GaAs quantumdots, which form at the intesection of three orthogonal quantum wells, allows a detailed investigation of the influence of coupling between almost identical zero-dimensional objects. In contrast to the inhomogeneously broadened quantum well and quantum wire signals originating from the complex twofold cleaved edge overgrowth structure, the photoluminescence spetrum of an individual quantumdot exhibits a single sharp line (full width at half maximum <70μeV) almost free of background signal. Microscopic photoluminescence excitation spectroscopy directly reveals the discreteness of the energy levels of the zero-dimensional structures and justifies the denomination "artificial atoms" for the quantumdots. It is further demonstrated that an "artifical molecule", characterized by the existence of bonding and antibonding states can be assembled from two of such "artificial atoms". The coupling strength between the "artificial atoms" is adjusted by the "interatomic" distance and is reflected in the energetic separation of the bonding and antibonding levels and the linewidths of the corresponding interband transitions.

This thesis presents a series of measurements investigating the spin physics of lateral quantumdots, defined electrostatically in the 2-D electron gas at the interface of a GaAs/AlGaAs heterostructure. The experiments span a range from open dots, where the leads of the dot carry at least one fully transmitting mode, to closed dots, where the leads are set to be tunnel barriers. For open dots, spin physics is inferred from measurements of conductance fluctuations; the effects of spin degeneracy in the orbital levels as well as a spin-orbit interaction are observed. In the closed dot measurements, ground state spin transitions as electrons are added to the dot may be determined from the motion of Coulomb blockade peaks in an in-plane magnetic field. In addition, this thesis demonstrates for the first time a direct measurement of the spin polarization of current emitted from a quantumdot, or a quantum point contact, during transport. These experiments make use of a spin-sensitive focusing geometry in which a quantum point contact serves as a spin analyzer for the mesoscopic device under test. Measurements are presented both in the open dot regime, where good agreement with theory is found, as well as the closed dot regime, where the data defies a simple theoretical explanation.

We will discuss the structural characterization of a system consisting of undoped self-assembled InSb quantumdots having a type II band alignment with the surrounding In0.53Ga0.47As matrix. This differs from systems using conventional type-I quantumdots that must be doped and that rely on intersubband transitions for infrared photoresponse. Type II dots grown in a superlattice structure combine the advantages of quantumdots (3-dimensional confinement) with the tunability and photovoltaic operation of the type II superlattice. We grew a high surface density of InSb quantumdots with a narrow distribution of sizes and shapes and free of dislocations within the body of the dots. The dots are relaxed due to an array of misfit dislocations confined at the basal dot/matrix interface. This makes burying the dots with InGaAs not feasible without generating dislocations due to the large dot/matrix lattice mismatch. We are experimenting with strain-compensating or graded strain overlayers to lower the lattice mismatch.

Quantum logic gates (QLGs) or other logic systems are based on quantum-dots (QD) with a stringent requirement of size uniformity. The QD are widely known building units for QLGs. The size control of QD is a critical issue in quantum-dot fabrication. The work presented here offers a new method to develop quantum-dots using a bio-template, called ferritin, that ensures QD production in uniform size of nano-scale proportion. The bio-template for uniform yield of QD is based on a ferritin protein that allows reconstitution of core material through the reduction and chelation processes. One of the biggest challenges for developing QLG is the requirement of ordered and uniform size of QD for arrays on a substrate with nanometer precision. The QD development by bio-template includes the electrochemical/chemical reconsitution of ferritins with different core materials, such as iron, cobalt, manganese, platinum, and nickel. The other bio-template method used in our laboratory is dendrimers, precisely defined chemical structures. With ferritin-templated QD, we fabricated the heptagonshaped patterned array via direct nano manipulation of the ferritin molecules with a tip of atomic force microscope (AFM). We also designed various nanofabrication methods of QD arrays using a wide range manipulation techniques. The precise control of the ferritin-templated QD for a patterned arrangement are offered by various methods, such as a site-specific immobilization of thiolated ferritins through local oxidation using the AFM tip, ferritin arrays induced by gold nanoparticle manipulation, thiolated ferritin positioning by shaving method, etc. In the signal measurements, the current-voltage curve is obtained by measuring the current through the ferritin, between the tip and the substrate for potential sweeping or at constant potential. The measured resistance near zero bias was 1.8 teraohm for single holoferritin and 5.7 teraohm for single apoferritin, respectively.

We analyze the optical bistability (OB) behavior in a multifold quantumdot (QD) molecule composed of five quantumdots controlled by the tunneling coupling. It is shown that the optical bistability can strongly be affected by the tunneling inter-dot coupling coefficients as well as detuning parameters. In addition, we find that the rate of an incoherent pump field has a leading role in modification of the OB threshold. We then generalize our analysis to the case of multifold quantumdot molecules where the number of the quantumdots is N (with a center dot and N-1 satellite dots). We compare the OB features that could occur in a multifold QD system consist of three (N= ), four (N=\\text{4} ), and five (N = 5) quantumdots. We realize that the OB threshold increases as the number of satellite QDs increases. Such controllable optical bistability in multiple QD molecules may provide some new possibilities for technological applications in optoelectronics and solid-state quantum information science.

The competition between the Kondo effect and the Ruderman-Kittel-Kasuya-Yoshida (RKKY) interaction is investigated in a double quantumdots system, coupled via a central open conducting region. A perpendicular magnetic field induces the formation of Landau Levels which in turn give rise to the so-called Kondo chessboard pattern in the transport through the quantumdots. The two quantumdots become therefore chirally coupled via the edge channels formed in the open conducting area. In regions where both quantumdots exhibit Kondo transport the presence of the RKKY exchange interaction is probed by an analysis of the temperature dependence. The thus obtained Kondo temperature of one dot shows an abrupt increase at the onset of Kondo transport in the other, independent of the magnetic field polarity, i.e. edge state chirality in the central region.

We present an all-optical implementation of quantum computation using semiconductor quantumdots. Quantum memory is represented by the spin of an excess electron stored in each dot. Two-qubit gates are realized by switching on trion-trion interactions between different dots. State selectivity is achieved via conditional laser excitation exploiting Pauli exclusion principle. Read out is performed via a quantum-jump technique. We analyze the effect on our scheme's performance of the main imperfections present in real quantumdots: exciton decay, hole mixing, and phonon decoherence. We introduce an adiabatic gate procedure that allows one to circumvent these effects and evaluate quantitatively its fidelity.

We investigate quantum transport through a quantumdot connected to source and drain leads and side coupled to a topological superconducting nanowire (Kitaev chain) sustaining Majorana end modes. Using a recursive Green's-function approach, we determine the local density of states of the system and find that the end Majorana mode of the wire leaks into the dot, thus, emerging as a unique dot level pinned to the Fermi energy ɛF of the leads. Surprisingly, this resonance pinning, resembling, in this sense, a "Kondo resonance," occurs even when the gate-controlled dot level ɛdot(Vg) is far above or far below ɛF. The calculated conductance G of the dot exhibits an unambiguous signature for the Majorana end mode of the wire: In essence, an off-resonance dot [ɛdot(Vg)≠ɛF], which should have G =0, shows, instead, a conductance e2/2h over a wide range of Vg due to this pinned dot mode. Interestingly, this pinning effect only occurs when the dot level is coupled to a Majorana mode; ordinary fermionic modes (e.g., disorder) in the wire simply split and broaden (if a continuum) the dot level. We discuss experimental scenarios to probe Majorana modes in wires via these leaked/pinned dot modes.

We study, in the framework of open quantum systems, the dynamics of quantum entanglement and quantum discord of two mutually independent circularly accelerated two-level atoms in interaction with a bath of fluctuating massless scalar fields in the Minkowski vacuum. We assume that the two atoms rotate synchronically with their separation perpendicular to the rotating plane. The time evolution of the quantum entanglement and quantum discord of the two-atom system is investigated. For a maximally entangled initial state, the entanglement measured by concurrence diminishes to zero within a finite time, while the quantum discord can either decrease monotonically to an asymptotic value or diminish to zero at first and then followed by a revival depending on whether the initial state is antisymmetric or symmetric. When both of the two atoms are initially excited, the generation of quantum entanglement shows a delayed feature, while quantum discord is created immediately. Remarkably, the quantum discord for such a circularly accelerated two-atom system takes a nonvanishing value in the steady state, and this is distinct from what happens in both the linear acceleration case and the case of static atoms immersed in a thermal bath.

Magnetization is induced in an ensemble of quantumdots, each charged with a single electron, when it is illuminated with a short circularly polarized light pulse that is resonant with the fundamental energy gap of the quantumdots. In this investigation, a quantum-mechanical model for the light-induced magnetization is presented. The phase of the magnetization precession as a function of the strength of the magnetic field in a Voigt geometry is in excellent agreement with experimental data measured on (In,Ga)As singly charged quantumdot ensembles. It is demonstrated that the precession of the hole in the trion plays a vital role because it determines the amplitude and phase of the magnetization precession. The model could also be easily extended to describe positively charged quantumdots. We also suggest that our theory, combined with measurements of the phase as a function of magnetic field, can be used as a technique to measure the resonant trion lifetime as a function of QD emission energy.

Quantumdot are semiconducting nanoparticles that have been used for decades in a variety of applications such as solar cells, LEDs and medical imaging. Their use in the last area, however, has been extremely limited despite their potential as revolutionary new biological labeling tools. Quantumdots are much brighter and more stable than conventional fluorophores, making them optimal for high resolution imaging and long term studies. Prior work in this area involves synthesizing and chemically conjugating quantumdots to molecules of interest in-house. However this method is both time consuming and prone to human error. Additionally, non-specific binding and nanoparticle aggregation currently prevent researchers from utilizing this system to its fullest capacity. Another critical issue that has not been addressed is determining the number of ligands bound to nanoparticles, which is crucial for proper interpretation of results. In this work, methods to label fixed cells using two types of chemically modified quantumdots are studied. Reproducible non-specific artifact labeling is consistently demonstrated if antibody-quantumdot conditions are less than optimal. In order to explain this, antibodies bound to quantumdots were characterized and quantified. While other groups have qualitatively characterized antibody functionalized quantumdots using TEM, AFM, UV spectroscopy and gel electrophoresis, and in some cases have reported calculated estimates of the putative number of total antibodies bound to quantumdots, no quantitative experimental results had been reported prior to this work. The chemical functionalization and characterization of quantumdot nanocrystals achieved in this work elucidates binding mechanisms of ligands to nanoparticles and allows researchers to not only translate our tools to studies in their own areas of interest but also derive quantitative results from these studies. This research brings ease of use and increased reliability to

Cavity quantum electrodynamics explores quantum optics at the most basic level of a single photon interacting with a single atom. We have been able to explore cavity QED in a condensed matter system by placing a double quantumdot (DQD) inside of a high quality factor microwave cavity. Our results show that measurements of the cavity field are sensitive to charge and spin dynamics in the DQD.[2,3] We can explore non-equilibrium physics by applying a finite source-drain bias across the DQD, which results in sequential tunneling. Remarkably, we observe a gain as large as 15 in the cavity transmission when the DQD energy level detuning is matched to the cavity frequency. These results will be discussed in the context of single atom lasing.[4] I will also describe recent progress towards reaching the strong-coupling limit in cavity-coupled Si DQDs. In collaboration with Manas Kulkarni, Yinyu Liu, Karl Petersson, George Stehlik, Jacob Taylor, and Hakan Tureci. We acknowledge support from the Sloan and Packard Foundations, ARO, DARPA, and NSF.

Colloidal quantumdots (CQDs) are free-standing nano-structures with chemically tunable electronic properties. This tunability offers intriguing possibilities for nano-electromechanical devices. In this work, we consider a nano-electromechanical nonvolatile memory (NVM) device incorporating a triple quantumdot (TQD) cluster. The device operation is based on a bias induced motion of a floating quantumdot (FQD) located between two bound quantumdots (BQDs). The mechanical motion is used for switching between two stable states, “ON” and “OFF” states, where ligand-mediated effective interdot forces between the BQDs and the FQD serve to hold the FQD in each stable position under zero bias. Considering realistic microscopic parameters, our quantum-classical theoretical treatment of the TQD reveals the characteristics of the NVM.

Semiconductor-based quantum structures integrated into mechanical resonators have emerged as a unique platform for generating entanglement between macroscopic phononic and mesocopic electronic degrees of freedom. A key challenge to realizing this is the ability to create and control the coupling between two vastly dissimilar systems. Here, such coupling is demonstrated in a hybrid device composed of a gate-defined quantumdot integrated into a piezoelectricity-based mechanical resonator enabling milli-Kelvin phonon states to be detected via charge fluctuations in the quantumdot. Conversely, the single electron transport in the quantumdot can induce a backaction onto the mechanics where appropriate bias of the quantumdot can enable damping and even current-driven amplification of the mechanical motion. Such electron transport induced control of the mechanical resonator dynamics paves the way towards a new class of hybrid semiconductor devices including a current injected phonon laser and an on-demand single phonon emitter. PMID:27063939

The electronic energy levels and optical transitions of a semiconductor quantumdot are subject to dynamics within the solid-state environment. In particular, fluctuating electric fields due to nearby charge traps or other quantumdots shift the transition frequencies via the Stark effect. The environment dynamics are mapped directly onto the fluorescence under resonant excitation and diminish the prospects of quantumdots as sources of indistinguishable photons in optical quantum computing. Here, we present an analysis of resonance fluorescence fluctuations based on photon counting statistics which captures the underlying time-averaged electric field fluctuations of the local environment. The measurement protocol avoids dynamic feedback on the electric environment and the dynamics of the quantumdot's nuclear spin bath by virtue of its resonant nature and by keeping experimental control parameters such as excitation frequency and external fields constant throughout. The method introduced here is experimentally undemanding. PMID:24810097

The electronic energy levels and optical transitions of a semiconductor quantumdot are subject to dynamics within the solid-state environment. In particular, fluctuating electric fields due to nearby charge traps or other quantumdots shift the transition frequencies via the Stark effect. The environment dynamics are mapped directly onto the fluorescence under resonant excitation and diminish the prospects of quantumdots as sources of indistinguishable photons in optical quantum computing. Here, we present an analysis of resonance fluorescence fluctuations based on photon counting statistics which captures the underlying time-averaged electric field fluctuations of the local environment. The measurement protocol avoids dynamic feedback on the electric environment and the dynamics of the quantumdot's nuclear spin bath by virtue of its resonant nature and by keeping experimental control parameters such as excitation frequency and external fields constant throughout. The method introduced here is experimentally undemanding. PMID:24810097

Semiconductor-based quantum structures integrated into mechanical resonators have emerged as a unique platform for generating entanglement between macroscopic phononic and mesocopic electronic degrees of freedom. A key challenge to realizing this is the ability to create and control the coupling between two vastly dissimilar systems. Here, such coupling is demonstrated in a hybrid device composed of a gate-defined quantumdot integrated into a piezoelectricity-based mechanical resonator enabling milli-Kelvin phonon states to be detected via charge fluctuations in the quantumdot. Conversely, the single electron transport in the quantumdot can induce a backaction onto the mechanics where appropriate bias of the quantumdot can enable damping and even current-driven amplification of the mechanical motion. Such electron transport induced control of the mechanical resonator dynamics paves the way towards a new class of hybrid semiconductor devices including a current injected phonon laser and an on-demand single phonon emitter.

Semiconductor-based quantum structures integrated into mechanical resonators have emerged as a unique platform for generating entanglement between macroscopic phononic and mesocopic electronic degrees of freedom. A key challenge to realizing this is the ability to create and control the coupling between two vastly dissimilar systems. Here, such coupling is demonstrated in a hybrid device composed of a gate-defined quantumdot integrated into a piezoelectricity-based mechanical resonator enabling milli-Kelvin phonon states to be detected via charge fluctuations in the quantumdot. Conversely, the single electron transport in the quantumdot can induce a backaction onto the mechanics where appropriate bias of the quantumdot can enable damping and even current-driven amplification of the mechanical motion. Such electron transport induced control of the mechanical resonator dynamics paves the way towards a new class of hybrid semiconductor devices including a current injected phonon laser and an on-demand single phonon emitter. PMID:27063939

Colloidal quantumdots (CQDs) are free-standing nano-structures with chemically tunable electronic properties. This tunability offers intriguing possibilities for nano-electromechanical devices. In this work, we consider a nano-electromechanical nonvolatile memory (NVM) device incorporating a triple quantumdot (TQD) cluster. The device operation is based on a bias induced motion of a floating quantumdot (FQD) located between two bound quantumdots (BQDs). The mechanical motion is used for switching between two stable states, "ON" and "OFF" states, where ligand-mediated effective interdot forces between the BQDs and the FQD serve to hold the FQD in each stable position under zero bias. Considering realistic microscopic parameters, our quantum-classical theoretical treatment of the TQD reveals the characteristics of the NVM.

A model to determine the electronic structure of self-assembled quantum arbitrarily shaped dots is applied. This model is based principally on constant effective mass and constant potentials of the barrier and quantumdot material. An analysis of the different parameters of this model is done and compared with those which take into account the variation of confining potentials, bands, and effective masses due to strain. The results are compared with several spectra reported in literature. By considering the symmetry, the computational cost is reduced with respect to other methods in literature. In addition, this model is not limited by the geometry of the quantumdot. PMID:15740390

InAs quantumdots are introduced into resonant tunneling diodes to study the electronic transport behavior, and a wide bistability (ΔV ~ 0.8 V) is observed in the negative differential resistance region. Based on an analytic model, we attribute the observed distinct bistability of a resonant tunneling diodes with quantumdots to the feedback dependence of energy of the electron-storing quantumdots on the tunneling current density. Meanwhile, we find that this wide bistable region can be modulated sensitively by light illumination and becomes narrower with increasing light intensity. Our results suggest that the present devices can be potentially used as sensitive photodetectors in optoelectronic fields.

Microstructured optical fibers containing quantumdots scintillation material comprised of zinc sulfide nanocrystals doped with magnesium sulfide are presented. These quantumdots are applied inside the microstructured optical fibers using capillary action. The x-ray photon counts of these fibers are compared to the output of a collimated CdTe solid state detector over an energy range from 10 to 40 keV. The results of the fiber light output and associated effects of an acrylate coating and the quantumdot application technique are discussed.

The recent availability of nanostructured materials has resulted in an explosion of research focused on their unique optical, thermal, mechanical and magnetic properties. Optical imagining, magnetic enhancement of contrast and drug delivery capabilities make the nanoparticles of special interest in biomedical applications. These materials have been involved in the development of theranostics—a new field of medicine that is focused on personalized tests and treatment. It is likely that multimodal nanomaterials will be responsible for future diagnostic advances in medicine. Quantumdots (QD) are nanoparticles which exhibit luminescence either through the formation of three-dimensional excitons or excitations of the impurities. The excitonic luminescence can be tuned by changing the size (the smaller the size, the higher the frequency). QDs are usually made of semiconducting materials. Unlike fluorescent proteins and organic dyes, QDs resist photobleaching, allow for multi-wavelength excitations and have narrow emission spectra. The techniques to make QDs are cheap and surface modifications and functionalizations can be implemented. Importantly, QDs could be synthesized to exhibit useful optomagnetic properties and, upon functionalization with an appropriate biomolecule, directed towards a pre-selected target for diagnostic imaging and photodynamic therapy. This special issue on Quantumdots in Biology is focused on recent research in this area. It starts with a topical review by Sreenivasan et al on various physical mechanisms that lead to the QD luminescence and on using wavelength shifts for an improvement in imaging. The next paper by Szczepaniak et al discusses nanohybrids involving QDs made of CdSe coated by ZnS and combined covalently with a photosynthetic enzyme. These nanohybrids are shown to maintain the enzymatic activity, however the enzyme properties depend on the size of a QD. They are proposed as tools to study photosynthesis in isolated

Wet electron microscopy (EM) is a new imaging method with the potential to allow higher spatial resolution of samples. In contrast to most EM methods, it requires little time to perform and does not require complicated equipment or difficult steps. We used this method on a common murine macrophage cell line, IC-21, in combination with various stains and preparations, to collect high resolution images of the actin cytoskeleton. Most importantly, we demonstrated the use of quantumdots in conjunction with this technique to perform light/electron correlation microscopy. We found that wet EM is a useful tool that fits into a niche between the simplicity of light microscopy and the high spatial resolution of EM. PMID:16989089

Successes in biomedical research and state-of-the-art medicine have undoubtedly improved the quality of life. However, a number of diseases, such as cancer, immunodeficiencies, and neurological disorders, still evade conventional diagnostic and therapeutic approaches. A transformation towards personalized medicine may help to combat these diseases. For this, identification of disease molecular fingerprints and their association with prognosis and targeted therapy must become available. Quantumdots (QDs), semiconductor nanocrystals with unique photo-physical properties, represent a novel class of fluorescence probes to address many of the needs of personalized medicine. This review outlines the properties of QDs that make them a suitable platform for advancing personalized medicine, examines several proof-of-concept studies showing utility of QDs for clinically relevant applications, and discusses current challenges in introducing QDs into clinical practice. PMID:20161004

We propose a simple approach to study the conductance through an array of N interacting quantumdots, weakly coupled to metallic leads. Using a mapping to an effective site which describes the low-lying excitations and a slave-boson representation in the saddle-point approximation, we calculated the conductance through the system. Explicit results are presented for N=1 and N=3 : a linear array and an isosceles triangle. For N=1 in the Kondo limit, the results are in very good agreement with previous results obtained with numerical renormalization group. In the case of the linear trimer for odd N , when the parameters are such that electron-hole symmetry is induced, we obtain perfect conductance G0=2e2/h . The validity of the approach is discussed in detail.

Results of studies on optical properties of low toxicity quantumdots (QDs) obtained from copper doped zinc sulfate are discussed in the paper. The effect of copper admixture concentration and solution pH on the fluorescence emission intensity of QDs was investigated. Quenching of QDs fluorescence by oxygen was reported and removal of the oxygen from the environment by two methods was described. In the chemical method oxygen was eliminated by adding sodium sulfite, in the other method oxygen was removed from the solution using nitrogen gas. For elimination of oxygen by purging the solution with nitrogen the increase of fluorescence intensity with decreasing oxygen concentration obeyed Stern-Volmer equation indicating quenching. For the chemical method Stern-Volmer equation was not fulfilled. The fluorescence decays lifetimes were determined and the increase of mean lifetimes at the absence of oxygen support hypothesis that QDs fluorescence is quenched by oxygen.

We have developed a novel nanoparticulate luminescent probe with inherent signal amplification upon interaction with a targeted proteolytic enzyme. This construct may be useful for imaging in cancer detection and diagnosis. In this system, quantumdots (QDs) are bound to gold nanoparticles (AuNPs) via a proteolytically degradable peptide sequence to non-radiatively suppress luminescence. A 71% reduction in luminescence was achieved with conjugation of AuNPs to QDs. Release of AuNPs by peptide cleavage restores radiative QD photoluminescence. Initial studies observed a 52% rise in luminescence over 47 h of exposure to 0.2 mg/mL collagenase. These probes can be customized for targeted degradation simply by changing the sequence of the peptide linker. PMID:16039606

A synthetic route for preparation of inorganic WS(2) nanotube (INT)-colloidal semiconductor quantumdot (QD) hybrid structures is developed, and transient carrier dynamics on these hybrids are studied via transient photoluminescence spectroscopy utilizing several different types of QDs. Measurements reveal efficient resonant energy transfer from the QDs to the INT upon photoexcitation, provided that the QD emission is at a higher energy than the INT direct gap. Charge transfer in the hybrid system, characterized using QDs with band gaps below the INT direct gap, is found to be absent. This is attributed to the presence of an organic barrier layer due to the relatively long-chain organic ligands of the QDs under study. This system, analogous to carbon nanotube-QD hybrids, holds potential for a variety of applications, including photovoltaics, luminescence tagging and optoelectronics. PMID:22354096

Using far-field optical microscopy we report the first measurements of photoluminescence from single nanoparticle photocatalysts. Fluence-dependent luminescence is investigated from metal-semiconductor heterojunction quantumdot catalysts exposed to a variety of environments, ranging from gaseous argon to liquid water containing a selection of hole scavengers. The catalysts each exhibit characteristic nonlinear fluence dependence. From these structurally and environmentally sensitive trends, we disentangle the separate rate-determining steps in each particle across the very wide range of time scales, which follow the initial light absorption process. This information will significantly benefit the design of effective artificial photocatalytic systems for renewable direct solar-to-fuel energy conversion. PMID:23895591

This review introduces quantumdots (QDs) and explores their properties, synthesis, applications, delivery systems in biology, and their toxicity. QDs are one of the first nanotechnologies to be integrated with the biological sciences and are widely anticipated to eventually find application in a number of commercial consumer and clinical products. They exhibit unique luminescence characteristics and electronic properties such as wide and continuous absorption spectra, narrow emission spectra, and high light stability. The application of QDs, as a new technology for biosystems, has been typically studied on mammalian cells. Due to the small structures of QDs, some physical properties such as optical and electron transport characteristics are quite different from those of the bulk materials. PMID:22929008

We report on a short-wavelength, 'blue' spectral shift of the photoluminescence (PL) spectrum in CdSeTe/ZnS core/shell quantumdots (QDs) caused by bioconjugation with several monoclonal cancer-related antibodies (ABs). Scanning PL spectroscopy was performed on samples dried on solid substrates at various temperatures. The influence of the AB chemical origin on the PL spectral shift was observed. The QD-AB conjugation reaction was confirmed using the agarose gel electrophoresis technique. The spectral shift was strongly increased and the process facilitated when the samples were dried above room temperature. The PL spectroscopic mapping revealed a profile of the PL spectral shift across the dried QD-AB spot. A mechanism of the blue shift is attributed to changes in the QD electronic energy levels caused by a local stress applied to the bioconjugated QD.

The continued growth of mobile and interactive computing requires devices manufactured with low-cost processes, compatible with large-area and flexible form factors, and with additional functionality. We review recent advances in the design of electronic and optoelectronic devices that use colloidal semiconductor quantumdots (QDs). The properties of materials assembled of QDs may be tailored not only by the atomic composition but also by the size, shape, and surface functionalization of the individual QDs and by the communication among these QDs. The chemical and physical properties of QD surfaces and the interfaces in QD devices are of particular importance, and these enable the solution-based fabrication of low-cost, large-area, flexible, and functional devices. We discuss challenges that must be addressed in the move to solution-processed functional optoelectronic nanomaterials. PMID:27563099

Nuclear spins play a major role in the dynamics of spin qubits in III-V semiconductor quantumdots. Although the hyperfine interaction between nuclear and electron (or hole) spins is typically viewed as the leading source of decoherence in these qubits, understanding how to experimentally control the nuclear spin polarization can not only ameliorate this problem, but in fact turn the nuclear spins into a valuable resource for quantum computing. Beyond extending decoherence times, control of this polarization can enable universal quantum computation as shown in singlet-triplet qubits and, in addition, offers the possibility of repurposing the nuclear spins into a robust quantum memory. In, we took a first step toward taking advantage of this resource by developing a general, fully quantum theory of non-unitary electron-nuclear spin dynamics with a periodic train of delta-function pulses as the external control driving the electron spin. Here, we extend this approach to other types of controls and further expand on the predictions and physical insights that emerge from the theory.

We study the H3×N hybrid entanglement between charge and vibrational modes in a triple-quantum-dot shuttle system. Three quantumdots are linearly connected, with the outer dots fixed and the central dot oscillating, described as a quantum harmonic oscillator with oscillation modes that are entangled with the electronic states of the quantumdots. The entangled states are characterized by the Schmidt number as a function of the parameters of the system: detuning and inverse tunneling length. We show that at steady state, as a function of detuning, the excited states of lower energy present Bell-type entanglement 2×N, with the participation of two quantumdots, while the more energetic excited states present 3×N entanglement, with the participation of three quantumdots. In the stationary regime, we find qualitative relationships between the maxima of the electronic current and the Schmidt number. Also, the time evolution of the degree of entanglement for a particular initial condition is studied in the presence of a time-dependent electric field and we evaluate the effects on entanglement of the condition of coherent destruction of tunneling.

We investigate the two-dimensional (2D) probe absorption in coupled quantumdots. It is found that, due to the position-dependent quantum interference effect, the 2D optical absorption spectrum can be easily controlled via adjusting the system parameters. Thus, our scheme may provide some technological applications in solid-state quantum communication.

In this article, the perturbation influences on optical characterization of quantumdot and quantumdot-quantum well (modified quantumdot) heteronanocrystal is investigated. The original aim of this article is to investigate the quantumdot-quantum well heteronanocrystal advantages and disadvantages, when used as a functionalized particle in biomedical applications. Therefore, all of the critical features of quantumdots are fundamentally studied and their influences on optical properties are simulated. For the first time, the perturbation effects on optical characteristics are observed in the quantumdot-quantum well heteronanocrystals by 8-band K.P theory. The impact of perturbation on optical features such as photoluminescence and shifting of wavelength is studied. The photoluminescence and operation wavelength of quantumdots play a vital role in biomedical applications, where their absorption and emission in biological assays are altered by shifting of wavelength. Furthermore, in biomedical applications, by tuning the emission wavelengths of the quantumdot into far-red and near-infrared ranges, non-invasive in-vivo imaging techniques have been easily developed. In this wavelength window, tissue absorption, scattering and auto-fluorescence intensities have minimum quantities; thus fixing or minimizing of wavelength shifting can be regarded as an important goal which is investigated in this work.

We have studied the polarized resolved photoluminescence in an n-type resonant tunneling diode (RTD) of GaAs/AlGaAs which incorporates a layer of InAs self-assembled quantumdots (QDs) in the center of a GaAs quantum well (QW). We have observed that the QD circular polarization degree depends on applied voltage and light intensity. Our results are explained in terms of the tunneling of minority carriers into the QW, carrier capture by InAs QDs and bias-controlled density of holes in the QW. PMID:21711647

One of the most promising systems for creating a working quantum computer is the triple quantumdots in a linear semiconductor. One of the biggest advantages is that we are able to perform Heisenberg exchange gates on the physical qubits. These exchanges are both fast and relatively low energy. Which means that they would be excellent for producing fast and accurate operations. In order to prevent leakage errors we use a 3 qubit DFS to encode a logical qubit. Here we determine the theoretical time dependent affects of applying the Heisenberg exchange gates in the DFS basis as well as the effect of applying multiple exchange gates at the same time. we also find that applying two heisenberg exchange gates at the same time is an effective way of implementing a leakage elimination operator.

We present the results of the simulation of a quantum neural network based on quantumdots using numerical method of path integral calculation. In the proposed implementation of the quantum neural network using an array of single-electron quantumdots with dipole-dipole interaction, the coherence is shown to survive up to 0.1 nanosecond in time and up to the liquid nitrogen temperature of 77K.We study the quantum correlations between the quantumdots by means of calculation of the entanglement of formation in a pair of quantumdots on the GaAs based substrate with dot size of 100 ÷ 101 nanometer and interdot distance of 101 ÷ 102 nanometers order.

Quantumdots grown by self-assembly process are typically constructed by 50,000 to 5,000,000 structural atoms which confine a small, countable number of extra electrons or holes in a space that is comparable in size to the electron wavelength. Under such conditions quantumdots can be interpreted as artificial atoms with the potential to be custom tailored to new functionality. In the past decade or so, these nanostructures have attracted significant experimental and theoretical attention in the field of nanoscience. The new and tunable optical and electrical properties of these artificial atoms have been proposed in a variety of different fields, for example in communication and computing systems, medical and quantum computing applications. Predictive and quantitative modeling and simulation of these structures can help to narrow down the vast design space to a range that is experimentally affordable and move this part of nanoscience to nano-Technology. Modeling of such quantumdots pose a formidable challenge to theoretical physicists because: (1) Strain originating from the lattice mismatch of the materials penetrates deep inside the buffer surrounding the quantumdots and require large scale (multi-million atom) simulations to correctly capture its effect on the electronic structure, (2) The interface roughness, the alloy randomness, and the atomistic granularity require the calculation of electronic structure at the atomistic scale. Most of the current or past theoretical calculations are based on continuum approach such as effective mass approximation or k.p modeling capturing either no or one of the above mentioned effects, thus missing some of the essential physics. The Objectives of this thesis are: (1) to model and simulate the experimental quantumdot topologies at the atomistic scale; (2) to theoretically explore the essential physics i.e. long range strain, linear and quadratic piezoelectricity, interband optical transition strengths, quantum confined

Controlling long-distance quantum correlations is central to quantum computation and simulation. In quantumdot arrays, experiments so far rely on nearest-neighbour couplings only, and inducing long-distance correlations requires sequential local operations. Here, we show that two distant sites can be tunnel-coupled directly. The coupling is mediated by virtual occupation of an intermediate site, with a strength that is controlled via the energy detuning of this site. It permits a single charge to oscillate coherently between the outer sites of a triple dot array without passing through the middle, as demonstrated through the observation of Landau-Zener-Stückelberg interference. The long-distance coupling significantly improves the prospects of fault-tolerant quantum computation using quantumdot arrays, and opens up new avenues for performing quantum simulations in nanoscale devices. PMID:23624695

Considering the environment protection, "green" materials are increasingly explored for photovoltaics. Here, we developed a kind of quantumdots solar cell based on nitrogen-doped carbon dots. The nitrogen-doped carbon dots were prepared by direct pyrolysis of citric acid and ammonia. The nitrogen-doped carbon dots' excitonic absorption depends on the N-doping content in the carbon dots. The N-doping can be readily modified by the mass ratio of reactants. The constructed "green" nitrogen-doped carbon dots solar cell achieves the best power conversion efficiency of 0.79 % under AM 1.5 G one full sun illumination, which is the highest efficiency for carbon dot-based solar cells. PMID:26781285

Considering the environment protection, "green" materials are increasingly explored for photovoltaics. Here, we developed a kind of quantumdots solar cell based on nitrogen-doped carbon dots. The nitrogen-doped carbon dots were prepared by direct pyrolysis of citric acid and ammonia. The nitrogen-doped carbon dots' excitonic absorption depends on the N-doping content in the carbon dots. The N-doping can be readily modified by the mass ratio of reactants. The constructed "green" nitrogen-doped carbon dots solar cell achieves the best power conversion efficiency of 0.79 % under AM 1.5 G one full sun illumination, which is the highest efficiency for carbon dot-based solar cells.

Medicinal applications of luminescent semiconductor quantumdots are of growing interest. In spite of the fact that their fabrication and imaging applications have been extensively investigated for the last decade, very little is documented on photodynamic action of quantumdots. In this study we demonstrate generation of singlet oxygen and other radical species upon exposure of quantumdots to blue light and therapeutic red light. Extent of radical production can be readily modified by antioxidants. Lay and scientific communities are two sites concerning potential hazards and enthusiastic applications of nanotechnology. Synthesis of quantumdots composed of less toxic materials is of great interest. A new candidate is a ubiquitous element carbon, which on nanoscale exhibits strong photoluminescence.

Graphene has been the subject of intense research in recent years due to its unique electrical, optical and mechanical properties. Furthermore, it is expected that quantumdots of graphene would make their way into devices due to their structure and composition which unify graphene and quantumdots properties. Graphene quantumdots (GQDs) are planar nano flakes with a few atomic layers thick and with a higher surface-to-volume ratio than spherical carbon dots (CDs) of the same size. We have developed a pulsed laser synthesis (PLS) method for the synthesis of GQDs that are soluble in water, measure 2–6 nm across, and are about 1–3 layers thick. They show strong intrinsic fluorescence in the visible region. The source of fluorescence can be attributed to various factors, such as: quantum confinement, zigzag edge structure, and surface defects. Confocal microscopy images of bacteria exposed to GQDs show their suitability as biomarkers and nano-probes in high contrast bioimaging.

Colloidal quantumdots have emerged as a material platform for low-cost high-performance optoelectronics. At the heart of optoelectronic devices lies the formation of a junction, which requires the intimate contact of n-type and p-type semiconductors. Doping in bulk semiconductors has been largely deployed for many decades, yet electronically active doping in quantumdots has remained a challenge and the demonstration of robust functional optoelectronic devices had thus far been elusive. Here we report an optoelectronic device, a quantumdot homojunction solar cell, based on heterovalent cation substitution. We used PbS quantumdots as a reference material, which is a p-type semiconductor, and we employed Bi-doping to transform it into an n-type semiconductor. We then combined the two layers into a homojunction device operating as a solar cell robustly under ambient air conditions with power conversion efficiency of 2.7%. PMID:24346430

Synthesis of semiconductor colloidal quantumdots by low-cost, solution-based methods has produced an abundance of basic science. Can these materials be transformed to high-performance light emitters to disrupt established photonics technologies, particularly semiconductor lasers?

We study numerically the optical properties of low-buckled silicene and AB-stacked bilayer graphene quantumdots subjected to an external electric field, which is normal to their surface. Within the tight-binding model, the optical absorption is calculated for quantumdots, of triangular and hexagonal shapes, with zigzag and armchair edge terminations. We show that in triangular silicene clusters with zigzag edges a rich and widely tunable infrared absorption peak structure originates from transitions involving zero energy states. The edge of absorption in silicene quantumdots undergoes red shift in the external electric field for triangular clusters, whereas blue shift takes place for hexagonal ones. In small clusters of bilayer graphene with zigzag edges the edge of absorption undergoes blue/red shift for triangular/hexagonal geometry. In armchair clusters of silicene blue shift of the absorption edge takes place for both cluster shapes, while red shift is inherent for both shapes of the bilayer graphene quantumdots.

We investigate the electron correlation in few-electron closed-shell atomic systems and similarly in few-electron quantumdots under weak confinement. As usual we start with restricted Hartree-Fock (HF) calculations and add electron correlation in steps in a series of approximations based on the single particle Green's function approach: (i) second-order Green function (GF); (ii) 2ph-Tamm-Dancoff approximation (TDA); and (iii) an extended version thereof which introduces ground-state correlation into the TDA. Our studies exhibit similarities and differences between weakly confined quantumdots and standard atomic systems. The calculations support the application of HF, GF, and TDA techniques in the modeling of three-dimensional quantumdot systems. The observed differences emphasize the significance of confinement and electronic features unique to quantumdots, such as the increased binding of electrons with higher angular momentum and thus - compared to atomic systems - modified shell-filling sequences.

A nanofluidic channel fabricated in fused silica with an approximately 500 nm square cross section was used to isolate, detect and identify individual quantumdot conjugates. The channel enables the rapid detection of every fluorescent entity in solution. A laser of selected wavelength was used to excite multiple species of quantumdots and organic molecules, and the emission spectra were resolved without significant signal rejection. Quantumdots were then conjugated with organic molecules and detected to demonstrate efficient multicolor detection. PCH was used to analyze coincident detection and to characterize the degree of binding. The use of a small fluidic channel to detect quantumdots as fluorescent labels was shown to be an efficient technique for multiplexed single molecule studies. Detection of single molecule binding events has a variety of applications including high throughput immunoassays.

A nanofluidic channel fabricated in fused silica with an approximately 500 nm square cross section was used to isolate, detect and identify individual quantumdot conjugates. The channel enables the rapid detection of every fluorescent entity in solution. A laser of selected wavelength was used to excite multiple species of quantumdots and organic molecules, and the emission spectra were resolved without significant signal rejection. Quantumdots were then conjugated with organic molecules and detected to demonstrate efficient multicolor detection. PCH was used to analyze coincident detection and to characterize the degree of binding. The use of a small fluidic channel to detect quantumdots as fluorescent labels was shown to be an efficient technique for multiplexed single molecule studies. Detection of single molecule binding events has a variety of applications including high throughput immunoassays.

We consider a triple quantumdot system in a triangular geometry with one of the dots connected to metallic leads. Using Wilson's numerical renormalization group method, we investigate quantum entanglement and its relation to the thermodynamic and transport properties in the regime where each of the dots is singly occupied on average, but with non-negligible charge fluctuations. It is shown that even in the regime of significant charge fluctuations the formation of the Kondo singlets induces switching between separable and perfectly entangled states. The quantum phase transition between unentangled and entangled states is analyzed quantitatively and the corresponding phase diagram is explained by exactly solvable spin model. In the framework of an effective model we also explain smearing of the entanglement transition for cases when the symmetry of the triple quantumdot system is relaxed.

This section describes recent progresses on the research of spin qubits realized in semiconductor quantumdot (QD) systems. After we argue the scheme of initialization and detection of individual spin states, we discuss the key idea of the universal gates constituted with QDs proposed by D. Loss and D. P. DiVincenzo. In order to achieve universal quantum gate operations, we need single qubit coherent manipulations and two qubit controlled-NOT or control-Z gates. For the first type of gate, instead of the standard rf magnetic field driven electron spin resonance (ESR), we proposed and implemented electric dipole induced spin resonance (EDSR), which has various advantages over ESR, including low dissipation, individual access to the spins and integrability. We describes recent progress in the fast Rabi oscillations. The second type of gate can be realized by the exchange coupling between nearby QDs. We also discuss the experiments combining single- and two-qubit operations. Finally, we argue the progress of the coupling of the spins in QDs with the "flying qubits", namely, photons of visible or microwave and itinerant electrons in the wave guides.

Colloidal quantumdots (CQDs) are an attractive material for optoelectronic applications because they combine flexible, low-cost solution-phase synthesis and processing with the potential for novel functionality arising from their nanostructure. Specifically, the bandgap of films composed of arrays of CQDs can be tuned via the quantum confinement effect for tailored spectral utilization. PbS-based CQDs can be tuned throughout the near and mid-infrared wavelengths and are a promising materials system for photovoltaic devices that harvest non-visible solar radiation. The performance of CQD solar cells is currently limited by an absorption-extraction compromise, whereby photon absorption lengths in the near infrared spectral regime exceed minority carrier diffusion lengths in the bulk films. Several light trapping strategies for overcoming this compromise and increasing the efficiency of infrared energy harvesting will be reviewed. A thin-film interference technique for creating multi-colored and transparent solar cells will be presented, and a discussion of designing plasmonic nanomaterials based on earth-abundant materials for integration into CQD solar cells is developed. The results indicate that it should be possible to achieve high absorption and color-tunability in a scalable nanomaterials system.

The assemblies of either quantumdots or magnetic nanoclusters are studied. It is shown that such assemblies can produce coherent radiation. A method is developed for solving the systems of nonlinear equations describing the dynamics of such assemblies. The method is shown to be general and applicable to systems of different physical nature. Despite mathematical similarities of dynamical equations, the physics of the processes for quantumdots and magnetic nanoclusters is rather different. In a quantumdot assembly, coherence develops due to the Dicke effect of dot interactions through the common radiation field. For a system of magnetic clusters, coherence in the spin motion appears due to the Purcell effect caused by the feedback action of a resonator. Self-organized coherent spin radiation cannot arise without a resonator. This principal difference is connected with the different physical nature of dipole forces between the objects. Effective dipole interactions between the radiating quantumdots, appearing due to photon exchange, collectivize the dot radiation. While the dipolar spin interactions exist from the beginning, yet before radiation, and on the contrary, they dephase spin motion, thus destroying the coherence of moving spins. In addition, quantumdot radiation exhibits turbulent photon filamentation that is absent for radiating spins.

Silicon quantumdot based qubits have emerged as an appealing approach to extending the success of GaAs spin based double quantumdot qubits. Research in this field is motivated by the promise of long spin coherence times, and within a MOS system the potential for variable carrier density, very small dot sizes, and CMOS compatibility. In this work, we will present results on the fabrication and transport properties of quantumdots in novel double gated Si MOS structures. Coulomb blockade is observed from single quantumdots with extracted charging energies up to an including 5meV. Observed dots were formed both from disorder within a quantum point contact, and through disorder free electrostatic confinement. Extracted capacitances, verified with 3D finite element simulations confirm the location of the disorder free dot to be within the designed lithographic structure. Distinctions will be made regarding the effects of feature sizes and sample processing. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Tunability of capacitive coupling in the Si double-quantum-dot system is discussed by changing the number of electrons in quantumdots (QDs), in which the QDs are fabricated using pattern-dependent oxidation (PADOX) of a Si nanowire and multi-fine-gate structure. A single QD formed by PADOX is divided into multiple QDs by additional oxidation through the gap between the fine gates. When the number of electrons occupying the QDs is large, the coupling capacitance increases gradually and almost monotonically with the number of electrons. This phenomenon is attributed to the gradual growth in the effective QD size due to the increase in the number of electrons in the QDs. On the other hand, when the number of electrons changes in the few-electron regime, the coupling capacitance irregularly changes. This irregularity can be observed even up to 40 electrons. This behavior is attributable the rough structure of Si nano-dots made by PADOX. This roughness is thought to induce complicated change in the electron wave function when an electron is added to or subtracted from a QD.

We investigate a QuantumDot System, in which the transconductance, in part, is due to spin coupling, with each dot subjected to a biasing voltage. When this system is housed in a QED cavity, the cavity dot coupling alters the spin coupling of the coupled dots significantly via the Purcell Effect. In this paper we show the extent to which one can control the various coupling parameters: the inter dot coupling, the individual dots coupling with the cavity and the coupled dots coupling with the cavity as a single entity. We show that the dots coupled to each other and to the cavity, the spin transport can be controlled selectively. We derive the conditions for such control explicitly. Further, we discuss the Quantum phase transition effects due to the charge and spin transport through the dots. The electron transport through the dots, electron-electron spin interaction and the electron-photon interaction are treated using the Non-equilibrium Green's Function Formalism. http://publish.aps.org/search/field/author/Trif_Mircea (Trif Mircea), http://publish.aps.org/search/field/author/Golovach_Vitaly_N (Vitaly N. Golovach), and http://publish.aps.org/search/field/author/Loss_Daniel (Daniel Loss), Phys. Rev. B 75, 085307 (2007)

This review examines axisymmetric-cavity quantum-dot microlasers whose emission spectrum is determined by whisperinggallery modes. We describe the possible designs, fabrication processes and basic characteristics of the microlasers and demonstrate the possibility of lasing at temperatures above 100 °C. The feasibility of creating multichannel optical sources based on a combination of a broadband quantum-dot laser and silicon microring modulators is discussed. (review)

We measure the ultrafast optical response of lithographically defined quantumdot amplifiers at 40 K. Recovery of the gain mostly occurs in less than 1 picosecond, with some longer-term transients attributable to carrier heating. Recovery of the absorption proceeds on a much longer timescale, representative of relaxation between quantumdot levels and carrier recombination. We also measure transparency current-density in these devices.

Quantumdots are ultra-small bits of semiconductor matter that can be synthesized with nearly atomic precision via modern methods of colloidal chemistry. Their emission color can be tuned by simply varying their dimensions. Color tunability is combined with high emission efficiencies approaching 100 percent. These properties have recently become the basis of a new technology – quantumdot displays – employed, for example, in the newest generation of e-readers and video monitors.

Kondo tunneling reveals hidden SO(n) dynamical symmetries of evenly occupied quantumdots. As is exemplified for an experimentally realizable triple quantumdot in parallel geometry, the possible values n=3,4,5,7 can be easily tuned by gate voltages. Following construction of the corresponding o(n) algebras, scaling equations are derived and Kondo temperatures are calculated. The symmetry group for a magnetic field induced anisotropic Kondo tunneling is SU(2) or SO(4). PMID:12366008

In this paper, we develop a theory for discrete solitons interaction with a string of coupled quantumdots in view of the local field effects. Discrete nonlinear Schrodinger (DNLS) equations are used to describe the dynamics of the string. Numerical calculations are carried out and results are analyzed with the help of matlab software. With the help of numerical solutions we demonstrate that in the quantumdots string, Rabi oscillations (RO) are self trapped into stable bright Rabi solitons. The Rabi oscillations in different types of nanostructures have potential applications to the elements of quantum logic and quantum memory.

Quantum switch is a primitive element in quantum network communication. In contrast to previous switch schemes on one degree of freedom (DOF) of quantum systems, we consider controlled switches of photon system with two DOFs. These controlled photon switches are constructed by exploring the optical selection rules derived from the quantum-dot spins in one-sided optical microcavities. Several double controlled-NOT gate on different joint systems are greatly simplified with an auxiliary DOF of the controlling photon. The photon switches show that two DOFs of photons can be independently transmitted in quantum networks. This result reduces the quantum resources for quantum network communication. PMID:26095049

This paper presents briefly the history of emission study in Si quantumdots (QDs) in the last two decades. Stable light emission of Si QDs and NCs was observed in the spectral ranges: blue, green, orange, red and infrared. These PL bands were attributed to the exciton recombination in Si QDs, to the carrier recombination through defects inside of Si NCs or via oxide related defects at the Si/SiOx interface. The analysis of recombination transitions and the different ways of the emission stimulation in Si QD structures, related to the element variation for the passivation of surface dangling bonds, as well as the plasmon induced emission and rare earth impurity activation, have been presented. The different applications of Si QD structures in quantum electronics, such as: Si QD light emitting diodes, Si QD single union and tandem solar cells, Si QD memory structures, Si QD based one electron devices and double QD structures for spintronics, have been discussed as well. Note the significant worldwide interest directed toward the silicon-based light emission for integrated optoelectronics is related to the complementary metal-oxide semiconductor compatibility and the possibility to be monolithically integrated with very large scale integrated (VLSI) circuits. The different features of poly-, micro- and nanocrystalline silicon for solar cells, that is a mixture of both amorphous and crystalline phases, such as the silicon NCs or QDs embedded in a α-Si:H matrix, as well as the thin film 2-cell or 3-cell tandem solar cells based on Si QD structures have been discussed as well. Silicon NC based structures for non-volatile memory purposes, the recent studies of Si QD base single electron devices and the single electron occupation of QDs as an important component to the measurement and manipulation of spins in quantum information processing have been analyzed as well.

In this thesis, we explore several aspects of both the software and hardware of quantum computation. First, we examine the computational power of multi-particle quantum random walks in terms of distinguishing mathematical graphs. We study both interacting and non-interacting multi-particle walks on strongly regular graphs, proving some limitations on distinguishing powers and presenting extensive numerical evidence indicative of interactions providing more distinguishing power. We then study the recently proposed adiabatic quantum algorithm for Google PageRank, and show that it exhibits power-law scaling for realistic WWW-like graphs. Turning to hardware, we next analyze the thermal physics of two nearby 2D electron gas (2DEG), and show that an analogue of the Coulomb drag effect exists for heat transfer. In some distance and temperature, this heat transfer is more significant than phonon dissipation channels. After that, we study the dephasing of two-electron states in a single silicon quantumdot. Specifically, we consider dephasing due to the electron-phonon coupling and charge noise, separately treating orbital and valley excitations. In an ideal system, dephasing due to charge noise is strongly suppressed due to a vanishing dipole moment. However, introduction of disorder or anharmonicity leads to large effective dipole moments, and hence possibly strong dephasing. Building on this work, we next consider more realistic systems, including structural disorder systems. We present experiment and theory, which demonstrate energy levels that vary with quantumdot translation, implying a structurally disordered system. Finally, we turn to the issues of valley mixing and valley-orbit hybridization, which occurs due to atomic-scale disorder at quantum well interfaces. We develop a new theoretical approach to study these effects, which we name the disorder-expansion technique. We demonstrate that this method successfully reproduces atomistic tight-binding techniques

We report on measurements of the magnetoconductance of an open circular InGaAs quantumdot between 1.3 and 204 K. We observe two types of magnetoconductance fluctuations: universal conductance fluctuations (UCFs), and “focusing” fluctuations related to ballistic trajectories between openings. The electron phase coherence time extracted from UCFs amplitude is larger than in GaAs/AlGaAs quantumdots and follows a similar temperature dependence (between T-1 and T-2). Below 150 K, the characteristic length associated with “focusing” fluctuations shows a slightly different temperature dependence from that of the conductivity.

Quantumdots that emit in the near-infrared can be used in vivo to follow circulation, to target the reticuloendothelial system, and to map lymphatic drainage from normal tissues and tumors. We have explored the role of surface charge and passivation by polyethylene glycol in determining circulating lifetimes and sites of deposition. Use of long polyethylene glycol polymers increases circulating lifetime. Changing surface charge can partially direct quantumdots to the liver and spleen, or the lymph nodes. Quantumdots are cleared in the order liver > spleen > bone marrow > lymph nodes. Quantumdots retained by lymph nodes maintained fluorescence for two years, suggesting either that the coating is extremely stable or that some endosomes preserve quantumdot function. We also explored migration from tumors to sentinel lymph nodes using tumor models in mice; surface charge and size make little difference to transport from tumors. Antibody and Fab-conjugates of polymer-coated quantumdots failed to target tumors in vivo, probably because of size.

We present energy absorption and interference in a quantumdot-metallic nanosphere system embedded on a dielectric substrate. A control field is applied to induce dipole moments in the nanosphere and the quantumdot, and a probe field is applied to monitor absorption. Dipole moments in the quantumdot or the metal nanosphere are induced, both by the external fields and by each other's dipole fields. Thus, in addition to direct polarization, the metal nanosphere and the quantumdot will sense one another via the dipole-dipole interaction. The density matrix method was used to show that the absorption spectrum can be split from one peak to two peaks by the control field, and this can also be done by placing the metal sphere close to the quantumdot. When the two are extremely close together, a self-interaction in the quantumdot produces an asymmetry in the absorption peaks. In addition, the fluorescence efficiency can be quenched by the addition of a metal nanosphere. This hybrid system could be used to create ultra-fast switching and sensing nanodevices.

The electron charging behavior of self assembled epitaxial Ge quantumdots on Si(100) grown using molecular beam epitaxy has been studied. Ge quantumdots encapsulated in n-type Si matrix were incorporated into Schottky diodes to investigate their charging behavior using capacitance-voltage measurements. These experimental results were interpreted in the context of theoretical models to assess the degree of charge localization to the dot. Experiments involving Ge quantumdot growth, growth of Sb-doped Si and morphological evolution during encapsulation of the Ge dots during Si overgrowth were performed in order to optimize the conditions for obtaining distinct Ge quantumdot morphologies. This investigation included finding a suitable method to minimize Sb segregation while maintaining good dot epitaxy and overall crystal quality. Holes are confined to the Ge dots for which the valence band offsets are large (˜650 meV). Electrons are confined to the strained Si regions adjacent to the Ge quantumdots which have relatively smaller confinement potentials (˜100--150 meV). Experimentally, it was found that but and pyramid clusters in the range from 20--40 nm in diameter confine ˜1electron per dot while dome clusters in the range from 60--80 nm diameter confine ˜6--8 electrons per dot. Theoretical simulations predict that similar pyramid structures confine ˜0.4 electrons per dot and dome structures confine ˜2.2--3 electrons per dot. Even though the theory and the experimental results disagree due to various uncertainties and approximations, the ratio between theory and experiment agree remarkably well for both island types. We also investigated constructive three-dimensional nanolithography. Nanoscale Au rich dots and pure Ge dots were deposited on SiO2 and Si3N4 substrates by decomposing adsorbed precursors using a focused electron beam in an environmental transmission electron microscope. Dimethyl acetylacetonate gold was used for Au and digermane was used to

We predict that the charge Kondo effect appears in a triangular triple quantumdot. The system has two-fold degenerate ground-state charge configurations, interdot Coulomb interactions, lead-dot electron tunnelings, but no interdot electron tunneling. We show, using bosonization and refermionization, that the system is described by the anisotropic Kondo model. The anisotropy can be tuned by changing lead-dot electron tunneling strength, which allows one to experimentally explore the transition between the ferromagnetic non-Fermi liquid and antiferromagnetic Kondo phases in the Kondo phase diagram. Using numerical renormalization group method, we demonstrate that the transition is manifested in electron conductances through the dot.

Self-assembled InGaAs quantumdot chains were grown using a modified Stranski-Krastanov method in which the InGaAs layer is deposited under a low growth temperature and high arsenic overpressure, which suppresses the formation of dots until a later annealing process. The dots are capped with a 100 nm GaAs layer. Three samples, having three different annealing temperatures of 460°C, 480°C, and 500°C, were studied by transmission electron microscopy. Results indicate two distinct types of dot formation processes: dots in the 460°C and 480°C samples form from platelet precursors in a one-to-one ratio whereas the dots in the sample annealed at 500°C form through the strain-driven self-assembly process, and then grow larger via an additional Ostwald ripening process whereby dots grow into larger dots at the expense of smaller seed islands. There are consequently significant morphological differences between the two types of dots, which explain many of the previously-reported differences in optical properties. Moreover, we also report evidence of indium segregation within the dots, with little or no indium intermixing between the dots and the surrounding GaAs barrier.

Self-assembled InGaAs quantumdot chains were grown using a modified Stranski-Krastanov method in which the InGaAs layer is deposited under a low growth temperature and high arsenic overpressure, which suppresses the formation of dots until a later annealing process. The dots are capped with a 100 nm GaAs layer. Three samples, having three different annealing temperatures of 460°C, 480°C, and 500°C, were studied by transmission electron microscopy. Results indicate two distinct types of dot formation processes: dots in the 460°C and 480°C samples form from platelet precursors in a one-to-one ratio whereas the dots in the sample annealed at 500°C form through the strain-driven self-assembly process, and then grow larger via an additional Ostwald ripening process whereby dots grow into larger dots at the expense of smaller seed islands. There are consequently significant morphological differences between the two types of dots, which explain many of the previously-reported differences in optical properties. Moreover, we also report evidence of indium segregation within the dots, with little or no indium intermixing between the dots and the surrounding GaAs barrier.

We investigate the interplay between long-range and direct photon-assisted transport in a triple quantumdot chain where local ac voltages are applied to the outer dots. We propose the phase difference between the two ac voltages as an external parameter, which can be easily tuned to manipulate the current characteristics. For gate voltages in phase opposition we find quantum destructive interferences analogous to the interferences in closed-loop undriven triple dots. As the voltages oscillate in phase, interferences between multiple paths give rise to dark states. Those totally cancel the current, and could be experimentally resolved.

We propose a simple setup of three coupled quantumdots in the Coulomb blockade regime as a source for spatially separated currents of spin-entangled electrons. The entanglement originates from the singlet ground state of a quantumdot with an even number of electrons. To preserve the entanglement of the electron pair during its extraction to the drain leads, the electrons are transported through secondary dots. This prevents one-electron transport by energy mismatch, while joint transport is resonantly enhanced by conservation of the total two-electron energy. PMID:12731992

We report on the observation of single-photon superradiance from an exciton in a semiconductor quantumdot. The confinement by the quantumdot is strong enough for it to mimic a two-level atom, yet sufficiently weak to ensure superradiance. The electrostatic interaction between the electron and the hole comprising the exciton gives rise to an anharmonic spectrum, which we exploit to prepare the superradiant quantum state deterministically with a laser pulse. We observe a fivefold enhancement of the oscillator strength compared to conventional quantumdots. The enhancement is limited by the base temperature of our cryostat and may lead to oscillator strengths above 1000 from a single quantum emitter at optical frequencies. PMID:27152804

We report on the observation of single-photon superradiance from an exciton in a semiconductor quantumdot. The confinement by the quantumdot is strong enough for it to mimic a two-level atom, yet sufficiently weak to ensure superradiance. The electrostatic interaction between the electron and the hole comprising the exciton gives rise to an anharmonic spectrum, which we exploit to prepare the superradiant quantum state deterministically with a laser pulse. We observe a fivefold enhancement of the oscillator strength compared to conventional quantumdots. The enhancement is limited by the base temperature of our cryostat and may lead to oscillator strengths above 1000 from a single quantum emitter at optical frequencies.

The spin of a single electron is the canonical two-level quantum system. When isolated in a semiconductor quantumdot, a single electron spin provides a well-controlled and long-lived quantum bit. So far, two-qubit gates in this system have relied on the spin exchange interaction that arises when the wave functions of neighbouring electrons overlap. Furthermore, experimental demonstrations of controlled spin-exchange have been limited to 1D quantumdot arrays only. Here we explore several avenues for scaling beyond 1D arrays with nearest-neighbour coupling. First, we show that second-order tunnel processes allow for coherent spin-exchange between non-nearest neighbour quantumdots. The detuning of the intermediate quantumdot controls the frequency of the exchange-driven oscillations of the spins. Second, we demonstrate shuttling of electrons in quantumdot arrays preserving the spin projection for more than 500 hops. We use this technique to read out multiple spins in a way analogous to the operation of a CCD. Finally, we develop superconducting resonators that are resilient to magnetic field and with a predicted tenfold increase in vacuum electric field amplitudes. This makes coupling spin qubits via superconducting resonators in a circuit-QED approach a realistic possibility. Supported by ERC, FOM, NWO, IARPA, ARO, EU.

Architectures that would exploit the distinct characteristics of quantum-dot cellular automata (QCA) have been proposed for digital communication networks that connect advanced digital computing circuits. In comparison with networks of wires in conventional very-large-scale integrated (VLSI) circuitry, the networks according to the proposed architectures would be more compact. The proposed architectures would make it possible to implement complex interconnection schemes that are required for some advanced parallel-computing algorithms and that are difficult (and in many cases impractical) to implement in VLSI circuitry. The difficulty of implementation in VLSI and the major potential advantage afforded by QCA were described previously in Implementing Permutation Matrices by Use of QuantumDots (NPO-20801), NASA Tech Briefs, Vol. 25, No. 10 (October 2001), page 42. To recapitulate: Wherever two wires in a conventional VLSI circuit cross each other and are required not to be in electrical contact with each other, there must be a layer of electrical insulation between them. This, in turn, makes it necessary to resort to a noncoplanar and possibly a multilayer design, which can be complex, expensive, and even impractical. As a result, much of the cost of designing VLSI circuits is associated with minimization of data routing and assignment of layers to minimize crossing of wires. Heretofore, these considerations have impeded the development of VLSI circuitry to implement complex, advanced interconnection schemes. On the other hand, with suitable design and under suitable operating conditions, QCA-based signal paths can be allowed to cross each other in the same plane without adverse effect. In principle, this characteristic could be exploited to design compact, coplanar, simple (relative to VLSI) QCA-based networks to implement complex, advanced interconnection schemes. The proposed architectures require two advances in QCA-based circuitry beyond basic QCA-based binary

Semiconductor quantumdots have emerged as promising candidates for the implementation of quantum information processing, because they allow for a quantum interface between stationary spin qubits and propagating single photons. In the meantime, transition-metal dichalcogenide monolayers have moved to the forefront of solid-state research due to their unique band structure featuring a large bandgap with degenerate valleys and non-zero Berry curvature. Here, we report the observation of zero-dimensional anharmonic quantum emitters, which we refer to as quantumdots, in monolayer tungsten diselenide, with an energy that is 20-100 meV lower than that of two-dimensional excitons. Photon antibunching in second-order photon correlations unequivocally demonstrates the zero-dimensional anharmonic nature of these quantum emitters. The strong anisotropic magnetic response of the spatially localized emission peaks strongly indicates that radiative recombination stems from localized excitons that inherit their electronic properties from the host transition-metal dichalcogenide. The large ∼1 meV zero-field splitting shows that the quantumdots have singlet ground states and an anisotropic confinement that is most probably induced by impurities or defects. The possibility of achieving electrical control in van der Waals heterostructures and to exploit the spin-valley degree of freedom renders transition-metal-dichalcogenide quantumdots interesting for quantum information processing. PMID:25938570

Green emission ZnO quantumdots were synthesized by an ultrasonic microreactor. Ultrasonic radiation brought bubbles through ultrasonic cavitation. These bubbles built microreactor inside the microreactor. The photoluminescence properties of ZnO quantumdots synthesized with different flow rate, ultrasonic power and temperature were discussed. Flow rate, ultrasonic power and temperature would influence the type and quantity of defects in ZnO quantumdots. The sizes of ZnO quantumdots would be controlled by those conditions as well. Flow rate affected the reaction time. With the increasing of flow rate, the sizes of ZnO quantumdots decreased and the quantum yields first increased then decreased. Ultrasonic power changed the ultrasonic cavitation intensity, which affected the reaction energy and the separation of the solution. With the increasing of ultrasonic power, sizes of ZnO quantumdots first decreased then increased, while the quantum yields kept increasing. The effect of ultrasonic temperature on the photoluminescence properties of ZnO quantumdots was influenced by the flow rate. Different flow rate related to opposite changing trend. Moreover, the quantum yields of ZnO QDs synthesized by ultrasonic microreactor could reach 64.7%, which is higher than those synthesized only under ultrasonic radiation or only by microreactor. PMID:27245962

In our spin-injection light-emitting diodes, electrons are spin-polarized in a semimagnetic ZnMnSe spin aligner and then injected into InGaAs quantumdots. The resulting electron spin state can be read out by measuring the circular polarization state of the emitted light. Here, we resonantly excite the Mn 3d electron spin system with microwave pulses and perform time-resolved measurements of the spin dynamics. We find that we are able to control the spin polarization of the injected electrons on a microsecond timescale. This electron spin resonance induced spin control could be one of the ingredients required to utilize the quantumdot electrons or the Mn spins as qubits.

Devices to generate on-demand non-local spin entangled electron pairs have potential application as solid-state analogues of the entangled photon sources used in quantum optics. Recently, Andreev entanglers that use two quantumdots as filters to adiabatically split and separate the quasi-particles of Cooper pairs have shown efficient splitting through measurements of the transport charge but the spin entanglement has not been directly confirmed. Here we report measurements on parallel quantumdot Josephson junction devices allowing a Josephson current to flow due to the adiabatic splitting and recombination of the Cooper pair between the dots. The evidence for this non-local transport is confirmed through study of the non-dissipative supercurrent while tuning independently the dots with local electrical gates. As the Josephson current arises only from processes that maintain the coherence, we can confirm that a current flows from the spatially separated entangled pair. PMID:26130172

Devices to generate on-demand non-local spin entangled electron pairs have potential application as solid-state analogues of the entangled photon sources used in quantum optics. Recently, Andreev entanglers that use two quantumdots as filters to adiabatically split and separate the quasi-particles of Cooper pairs have shown efficient splitting through measurements of the transport charge but the spin entanglement has not been directly confirmed. Here we report measurements on parallel quantumdot Josephson junction devices allowing a Josephson current to flow due to the adiabatic splitting and recombination of the Cooper pair between the dots. The evidence for this non-local transport is confirmed through study of the non-dissipative supercurrent while tuning independently the dots with local electrical gates. As the Josephson current arises only from processes that maintain the coherence, we can confirm that a current flows from the spatially separated entangled pair. PMID:26130172

We demonstrate that the quantum coherence of mesoscopic very-high-n, n {approx} 305, Rydberg wave packets travelling along nearly circular orbits can be maintained on microsecond time scales corresponding to hundreds of classical orbital periods. The coherence is probed through collapses and revivals of periodic oscillations in the average electron position. The temporal interferences of spatially separated Schroedinger cat-like wave packets are also observed. A novel hybrid quantum-classical trajectory method is employed to simulate the wave packet dynamics.

We investigate theoretically the balance of the static magnetic and the dynamical photon forces in the electron transport through a quantumdot in a photon cavity with a single photon mode. The quantumdot system is connected to external leads and the total system is exposed to a static perpendicular magnetic field. We explore the transport characteristics through the system by tuning the ratio, \\hslash {ωγ}/\\hslash {ωc} , between the photon energy, \\hslash {ωγ} , and the cyclotron energy, \\hslash {ωc} . Enhancement in the electron transport with increasing electron–photon coupling is observed when \\hslash {ωγ}/\\hslash {ωc}>1 . In this case the photon field dominates and stretches the electron charge distribution in the quantumdot, extending it towards the contact area for the leads. Suppression in the electron transport is found when \\hslash {ωγ}/\\hslash {ωc}<1 , as the external magnetic field causes circular confinement of the charge density around the dot.

Solution-processed electronic and optoelectronic devices offer low cost, large device area, physical flexibility and convenient materials integration compared to conventional epitaxially grown, lattice-matched, crystalline semiconductor devices. Although the electronic or optoelectronic performance of these solution-processed devices is typically inferior to that of those fabricated by conventional routes, this can be tolerated for some applications in view of the other benefits. Here we report the fabrication of solution-processed infrared photodetectors that are superior in their normalized detectivity (D*, the figure of merit for detector sensitivity) to the best epitaxially grown devices operating at room temperature. We produced the devices in a single solution-processing step, overcoating a prefabricated planar electrode array with an unpatterned layer of PbS colloidal quantumdot nanocrystals. The devices showed large photoconductive gains with responsivities greater than 103AW-1. The best devices exhibited a normalized detectivity D* of 1.8 × 1013jones (1jones = 1cmHz1/2W-1) at 1.3µm at room temperature: today's highest performance infrared photodetectors are photovoltaic devices made from epitaxially grown InGaAs that exhibit peak D* in the 1012jones range at room temperature, whereas the previous record for D* from a photoconductive detector lies at 1011jones. The tailored selection of absorption onset energy through the quantum size effect, combined with deliberate engineering of the sequence of nanoparticle fusing and surface trap functionalization, underlie the superior performance achieved in this readily fabricated family of devices.

The study of nanometer sized semiconductor crystallites, also known as quantumdots (QDs), has seen rapid advancements in recent years in scientific disciplines ranging from chemistry, physics, biology, materials science, and engineering. QD materials of CdSe, ZnSe, InP, as well as many others, can be prepared in the size range of 1-10 nm producing uniform, nearly monodisperse materials that are typically coated with organic molecules [1-3]. The strength of charge carrier confinement, which dictates the size-dependent properties, in these QDs depends on the nature of the material and can be correlated to the Bohr radius for the system of interest. For instance, the Bohr radius for CdSe is {approx} 5 nm, while in the more covalent structure of InP, the Bohr radius approaches {approx} 10 nm. The study of CdSe QDs has been particularly extensive during the last decade because they exhibit unique and tunable optical properties and are readily synthesized with high-crystallinity and narrow size dispersions. Although the core electronic properties of CdSe are explained in terms of the quantum confinement model, experimental efforts to elucidate the surface structure of these materials have been limited. Typically, colloidal CdSe QDs are coated with an organic surfactant, which typically consists of an organo-phosphine, -thiol, or -amine, that has the function of energetically relaxing defect states via coordination to partially coordinated surface atoms. The organic surfactant also acts to enhance carrier confinement and prevent agglomeration of the particles. Chemically, it has been shown that the bonding of the surfactant to the CdSe QD occurs through Cd atoms resulting cleavage of the Se atoms and formation of a Cd-rich (i.e. non-stoichiometric) particle [5].

It is analytically shown that the both the charge carrier dynamics in quantumdots and their capture into the quantumdots from the matrix material have a significant effect on two-state lasing phenomenon in quantumdot lasers. In particular, the consideration of desynchronization in electron and hole capture into quantumdots allows one to describe the quenching of ground-state lasing observed at high injection currents both qualitatevely and quantitatively. At the same time, an analysis of the charge carrier dynamics in a single quantumdot allowed us to describe the temperature dependences of the emission power via the ground- and excited-state optical transitions of quantumdots.

We have studied the influence of the quantum-dot (QD) width and the quantum-dot conduction band (QD-CB) offset on the efficiency of quantum-dot intermediate band solar cells (QD-IBSCs). Simulation results demonstrate that with increasing QD-CB offset and decreasing QD width, the maximum efficiency is achieved. (laser applications and other topics in quantum electronics)

Using the tight-binding approach, we study the influence of a perpendicular magnetic field on the energy levels of hexagonal, triangular, and circular bilayer graphene (BLG) quantumdots (QDs) with zigzag and armchair edges. We obtain the energy levels for AB (Bernal)-stacked BLG QDs in both the absence and the presence of a perpendicular electric field (i.e., biased BLG QDs). We find different regions in the spectrum of biased QDs with respect to the crossing point between the lowest-electron and -hole Landau levels of a biased BLG sheet. Those different regions correspond to electron states that are localized at the center, edge, or corner of the BLG QD. Quantum Hall corner states are found to be absent in circular BLG QDs. The spatial symmetry of the carrier density distribution is related to the symmetry of the confinement potential, the position of zigzag edges, and the presence or absence of interlayer inversion symmetry.

The progression of atherosclerosis is associated with leukocyte infiltration within lesions. We describe a technique for the ex vivo imaging of cellular recruitment in atherogenesis which utilizes quantumdots (QD) to color-code different cell types within lesion areas. Spectrally distinct QD were coated with the cell-penetrating peptide maurocalcine to fluorescently-label immunomagnetically isolated monocyte/macrophages and T lymphocytes. QD-maurocalcine bioconjugates labeled both cell types with a high efficiency, preserved cell viability, and did not perturb native leukocyte function in cytokine release and endothelial adhesion assays. QD-labeled monocyte/macrophages and T lymphocytes were reinfused in an ApoE-/- mouse model of atherosclerosis and age-matched controls and tracked for up to four weeks to investigate the incorporation of cells within aortic lesion areas, as determined by oil red O (ORO) and immunofluorescence ex vivo staining. QD-labeled cells were visible in atherosclerotic plaques within two days of injection, and the two cell types colocalized within areas of subsequent ORO staining. Our method for tracking leukocytes in lesions enables high signal-to-noise ratio imaging of multiple cell types and biomarkers simultaneously within the same specimen. It also has great utility in studies aimed at investigating the role of distinct circulating leukocyte subsets in plaque development and progression.

Synthesis of new antibacterial agents is becoming increasingly important in light of the emerging antibiotic resistance. In the present study we report that electrochemically produced graphene quantumdots (GQD), a new class of carbon nanoparticles, generate reactive oxygen species when photoexcited (470 nm, 1 W), and kill two strains of pathogenic bacteria, methicillin-resistant Staphylococcus aureus and Escherichia coli. Bacterial killing was demonstrated by the reduction in number of bacterial colonies in a standard plate count method, the increase in propidium iodide uptake confirming the cell membrane damage, as well as by morphological defects visualized by atomic force microscopy. The induction of oxidative stress in bacteria exposed to photoexcited GQD was confirmed by staining with a redox-sensitive fluorochrome dihydrorhodamine 123. Neither GQD nor light exposure alone were able to cause oxidative stress and reduce the viability of bacteria. Importantly, mouse spleen cells were markedly less sensitive in the same experimental conditions, thus indicating a fairly selective antibacterial photodynamic action of GQD. PMID:24612819

Carbon quantumdots (C-QDs) are becoming a desirable alternative to metal-based QDs and dye probes owing to their high biocompatibility, low toxicity, ease of preparation, and unique photophysical properties. Herein, we describe fluorescence bioimaging of zebrafish using C-QDs as probe in terms of the preparation of C-QDs, zebrafish husbandry, embryo harvesting, and introduction of C-QDs into embryos and larvae by soaking and microinjection. The multicolor of C-QDs was validated with their imaging for zebrafish embryo. The distribution of C-QDs in zebrafish embryos and larvae were successfully observed from their fluorescence emission. the bio-toxicity of C-QDs was tested with zebrafish as model and C-QDs do not interfere to the development of zebrafish embryo. All of the results confirmed the high biocompatibility and low toxicity of C-QDs as imaging probe. The absorption, distribution, metabolism and excretion route (ADME) of C-QDs in zebrafish was revealed by their distribution. Our work provides the useful information for the researchers interested in studying with zebrafish as a model and the applications of C-QDs. The operations related zebrafish are suitable for the study of the toxicity, adverse effects, transport, and biocompatibility of nanomaterials as well as for drug screening with zebrafish as model. PMID:26135470

A direct current usually requires the application of a non-zero potential difference between source and drain, but on nanoscale systems (NSS) it is possible to obtain a non-zero current while the potential difference is zero. The effect is known as quantum charge pumping (QCP) and it is due to the interference provided by the existence of a time-dependent potential (TDP). QCP can be generated by a TDP in non-adiabatic limit. An example of this is a system composed by a ring with a dot embedded on it, under the application of an oscillating TDP. By the action of a magnetic field across the system, a pumped current is generated, since time reversal symmetry is broken. Decoherence is crucial, both from a scientific and technological point of view. In NSS it is expected that decoherence, among others things, decreases the QCP amplitude. In this context, we study what is the effect of a bath on the pumped current in our system. We find that for certain values of magnetic flux, the bath-system produce amplification of the pumped current.

Carbon group quantumdots (QDs) such as carbon, silicon and germanium, have potential for biomedical applications such as bio-imaging markers and drug delivery systems and are expected to demonstrate several advantages over conventional fluorescent QDs such as CdSe, especially in biocompatibility. We assessed biocompatibility of newly manufactured silicon QDs (Si-QDs), by means of both MTT assay and LDH assay for HeLa cells in culture and thereby detected the cellular toxicity by administration of high concentration of Si-QD (>1000 μg/mL), while we detected the high toxicity by administration of over 100 μg/mL of CdSe-QDs. As a hypothesis for the cause of the cellular toxicity, we measured oxy-radical generation from the QDs by means of luminol reaction method. We detected generation of oxy-radicals from the Si-QDs and those were decreased by radical scavenger such as superoxide dismutase (SOD) and N-acetyl cysteine (NAC). We concluded that the Si-QD application to cultured cells in high concentration led cell membrane damage by oxy-radicals and combination usage with radical scavenger is one of the answers.

The ability to incorporate a dopant element into silicon nanocrystals (NC) and quantumdots (QD) is one of the key technical challenges for the use of these materials in a number of optoelectronic applications. Unlike doping of traditional bulk semiconductor materials, the location of the doping element can be either within the crystal lattice (c-doping), on the surface (s-doping) or within the surrounding matrix (m-doping). A review of the various synthetic strategies for doping silicon NCs and QDs is presented, concentrating on the efficacy of the synthetic routes, both in situ and post synthesis, with regard to the structural location of the dopant and the doping level. Methods that have been applied to the characterization of doped NCs and QDs are summarized with regard to the information that is obtained, in particular to provide researchers with a guide to the suitable techniques for determining dopant concentration and location, as well as electronic and photonic effectiveness of the dopant. PMID:26727507

The ability to incorporate a dopant element into silicon nanocrystals (NC) and quantumdots (QD) is one of the key technical challenges for the use of these materials in a number of optoelectronic applications. Unlike doping of traditional bulk semiconductor materials, the location of the doping element can be either within the crystal lattice (c-doping), on the surface (s-doping) or within the surrounding matrix (m-doping). A review of the various synthetic strategies for doping silicon NCs and QDs is presented, concentrating on the efficacy of the synthetic routes, both in situ and post synthesis, with regard to the structural location of the dopant and the doping level. Methods that have been applied to the characterization of doped NCs and QDs are summarized with regard to the information that is obtained, in particular to provide researchers with a guide to the suitable techniques for determining dopant concentration and location, as well as electronic and photonic effectiveness of the dopant.

Fluorescent nanoparticles, such as nanocrystal quantumdots (QDs), have potential to be applied to molecular biology and bioimaging, since some nanocrystals emit higher and longer lasting fluorescence than conventional organic probes do. Here we report an example of labeling immune cells by QDs. We collected splenic CD4 + T-lymphocyte and peritoneal macrophages from mice. Then cells were labeled with QDs. QDs are incorporated into the T-lymphocyte and macrophages immediately after addition and located in the cytoplasm via endocytosis pathway. The fluorescence of QDs held in the endosomes was easily detected for more than a week. In addition, T-lymphocytes labeled with QDs were stable and cell proliferation or cytokine production including IL-2 and IFN-γ was not affected. When QD-labeled T-lymphocytes were adoptively transferred intravenously to mice, they remained in the peripheral blood and spleen up to a week. Using QD-labeled peritoneal macrophages, we studied cell traffic during inflammation on viscera in peritoneum cavity. QD-labeled macrophages were transplanted into the peritoneum of the mouse, and colitis was induced by intracolonic injection of a hapten, trinitrobenzensulfonic acid. With the aid of stong signals of QDs, we found that macrophage accumuled on the inflammation site of the colon. These results suggested that fluorescent probes of QDs might be useful as bioimaging tools for tracing target cells in vivo.

Carbon quantumdots (C-QDs) are becoming a desirable alternative to metal-based QDs and dye probes owing to their high biocompatibility, low toxicity, ease of preparation, and unique photophysical properties. Herein, we describe fluorescence bioimaging of zebrafish using C-QDs as probe in terms of the preparation of C-QDs, zebrafish husbandry, embryo harvesting, and introduction of C-QDs into embryos and larvae by soaking and microinjection. The multicolor of C-QDs was validated with their imaging for zebrafish embryo. The distribution of C-QDs in zebrafish embryos and larvae were successfully observed from their fluorescence emission. the bio-toxicity of C-QDs was tested with zebrafish as model and C-QDs do not interfere to the development of zebrafish embryo. All of the results confirmed the high biocompatibility and low toxicity of C-QDs as imaging probe. The absorption, distribution, metabolism and excretion route (ADME) of C-QDs in zebrafish was revealed by their distribution. Our work provides the useful information for the researchers interested in studying with zebrafish as a model and the applications of C-QDs. The operations related zebrafish are suitable for the study of the toxicity, adverse effects, transport, and biocompatibility of nanomaterials as well as for drug screening with zebrafish as model. PMID:26135470

We examine electronic heat conduction via molecular complexes in the presence of local electron-phonon coupling effects. In off-resonance transport regime, even weak electron-phonon interactions lead to phonon-mediated changes of transport characteristics. In the nearly resonance conditions, the strong electron-phonon coupling reduces the height of the main conductance peak, generating additional satellites (phonon sidebands) in transport characteristics and shifting molecular energy spectrum via reorganization (polaron) energy. In the past, it was shown that inclusion of electron-phonon coupling effects into computational scheme reduces discrepancy between theoretical results and experimental data. The aim of this project is to study electron-phonon coupling effects on electronic heat transfer at molecular level. For that purpose, we use non-perturbative computational scheme based on inelastic version of Landauer formula, where the Green's functions technique combined with polaron transformation was used to calculate multi-channel transmission probability function, while accessibility of individual conduction channels is governed by Boltzmann statistics. Our analysis is based on the hypothesis that the dynamics created by electron-phonon interaction onto the molecular quantumdot asymmetrically connected to two thermal reservoirs will lead to thermal rectification effect. Our results will be discussed in a few aspects: electron-phonon coupling strength, phonon dispersion relationship, and heat fluxes generated by temperature difference as well as bias voltage.

Carbon quantumdots (C-QDs) are becoming a desirable alternative to metal-based QDs and dye probes owing to their high biocompatibility, low toxicity, ease of preparation, and unique photophysical properties. Herein, we describe fluorescence bioimaging of zebrafish using C-QDs as probe in terms of the preparation of C-QDs, zebrafish husbandry, embryo harvesting, and introduction of C-QDs into embryos and larvae by soaking and microinjection. The multicolor of C-QDs was validated with their imaging for zebrafish embryo. The distribution of C-QDs in zebrafish embryos and larvae were successfully observed from their fluorescence emission. the bio-toxicity of C-QDs was tested with zebrafish as model and C-QDs do not interfere to the development of zebrafish embryo. All of the results confirmed the high biocompatibility and low toxicity of C-QDs as imaging probe. The absorption, distribution, metabolism and excretion route (ADME) of C-QDs in zebrafish was revealed by their distribution. Our work provides the useful information for the researchers interested in studying with zebrafish as a model and the applications of C-QDs. The operations related zebrafish are suitable for the study of the toxicity, adverse effects, transport, and biocompatibility of nanomaterials as well as for drug screening with zebrafish as model.

Colloidal quantumdots (CQDs) are attractive materials for solar cells due to their low cost, ease of fabrication and spectral tunability. Progress in CQD photovoltaic technology over the past decade has resulted in power conversion efficiencies approaching 10%. In this review, we give an overview of this progress, and discuss limiting mechanisms and paths for future improvement in CQD solar cell technology.We briefly summarize nanoparticle synthesis and film processing methods and evaluate the optoelectronic properties of CQD films, including the crucial role that surface ligands play in materials performance. We give an overview of device architecture engineering in CQD solar cells. The compromise between carrier extraction and photon absorption in CQD photovoltaics is analyzed along with different strategies for overcoming this trade-off. We then focus on recent advances in absorption enhancement through innovative device design and the use of nanophotonics. Several light-trapping schemes, which have resulted in large increases in cell photocurrent, are described in detail. In particular, integrating plasmonic elements into CQD devices has emerged as a promising approach to enhance photon absorption through both near-field coupling and far-field scattering effects. We also discuss strategies for overcoming the single junction efficiency limits in CQD solar cells, including tandem architectures, multiple exciton generation and hybrid materials schemes. Finally, we offer a perspective on future directions for the field and the most promising paths for achieving higher device efficiencies.

In the framework of a microscopic model for intersubband gain from electrically pumped quantum-dot structures we investigate electrically pumped quantum-dots as active material for a mid-infrared quantum cascade laser. Our previous calculations have indicated that these structures could operate with reduced threshold current densities while also achieving a modal gain comparable to that of quantum well active materials. We study the influence of two important quantum-dot material parameters, here, namely inhomogeneous broadening and quantum-dot sheet density, on the performance of a proposed quantum cascade laser design. In terms of achieving a positive modal net gain, a high quantum-dot density canmore » compensate for moderately high inhomogeneous broadening, but at a cost of increased threshold current density. By minimizing quantum-dot density with presently achievable inhomogeneous broadening and total losses, significantly lower threshold densities than those reported in quantum-well quantum-cascade lasers are predicted by our theory.« less

InAsSb quantum-dot lasers near 2 micrometers were demonstrated in cw operation at room temperature with a threshold current density of 733 A,/cm(sup 2), output power of 3 mW/facet and a differential quantum efficiency of 13%.

The ability to electrically control spin dynamics in quantumdots makes them one of the most promising platforms for solid-state quantum-information processing. Minimizing the influence of the nuclear spin environment is an important step towards realizing such promise.

The integer quantum Hall effect in systems of semiconductor quantumdots and antidots is studied theoretically as a factor of temperature. It is established that the conditions for carrier localization in quantum-dot systems favor the observation of the quantum Hall effect at higher temperatures than in quantum-well systems. The obtained numerical results show that the fundamental plateau corresponding to the transition between the ground and first excited Landau levels can be retained up to a temperature of T ∼ 50 K, which is an order of magnitude higher than in the case of quantum wells. Implementation of the quantum Hall effect at such temperatures requires quantum-dot systems with controllable characteristics, including the optimal size and concentration and moderate geometrical and composition fluctuations. In addition, ordered arrangement is desirable, hence quantum antidots are preferable.

Rutile phase of SnO2 quantumdots of average size of 2.5 nm were synthesized at a growth temperature of 70 °C and characterized with XRD, TEM, FTIR and Raman analysis. The effective strain within the lattice of SnO2 quantumdots was calculated by Williamson-Hall method. The broad peaks in XRD as well as Raman spectra and the presence of Raman bands at 569 and 432 cm-1 are due to lower crystallinity of nanoparticles. The optical band gap of SnO2 quantumdots was increased to 3.75 eV attributed to the quantum size effect. SnO2 quantumdots were annealed in air atmosphere and the crystallite size of the particles increased with annealing temperature. Sunlight assisted photodegration property of SnO2 quantumdots was investigated with vanillin as a model system and it shows the photodegradation efficiency of 87%. The photoluminescence and photodegradation efficiency of nanocrystallite SnO2 decreases with increase of crystallite size contributed to the reduction in population of defects and surface area.

Semiconductor quantumdots, tiny light-emitting particles on thenanometer scale, are emerging as a new class of fluorescent labels for a broad range of molecular and cellular applications. In comparison with organic dyes and fluorescent proteins, they have unique optical and electronic properties such as size-tunable light emission, intense signal brightness, resistance to photobleaching, and broadband absorption for simultaneous excitation of multiple fluorescence colors. Here we report new advances in minimizing the hydrodynamic sizes of quantumdots using multidentate and multifunctional polymer coatings. A key finding is that a linear polymer containing grafted amine and thiol coordinating groups can coat nanocrystals and lead to a highly compact size, exceptional colloidal stability, strong resistance to photobleaching, and high fluorescence quantum yields. This has allowed a new generation of bright and stable quantumdots with small hydrodynamic diameters between 5.6 and 9.7 nm with tunable fluorescence emission from the visible (515 nm) to the near infrared (720 nm). These quantumdots are well suited for molecular and cellular imaging applications in which the nanoparticle hydrodynamic size needs to be minimized. Together with the novel properties of new strain-tunable quantumdots, these findings will be especially useful for multicolor and super-resolution imaging at the single-molecule level.

A quantum-dot hybrid qubit formed from three electrons in a double quantumdot has the potential for great speed, due to the presence of level crossings where the qubit becomes chargelike. Here, we show how to exploit the level crossings to implement fast pulsed gating. We develop one- and two-qubit dc quantum gates that are simpler than the previously proposed ac gates. We obtain closed-form solutions for the control sequences and show that the gates are fast (subnanosecond) and can achieve high fidelities.

We present a modulated microwave approach for quantum computing with qubits comprising three spins in a triple quantumdot. This approach includes single- and two-qubit gates that are protected against low-frequency electrical noise, due to an operating point with a narrowband response to high frequency electric fields. Furthermore, existing double quantumdot advances, including robust preparation and measurement via spin-to-charge conversion, are immediately applicable to the new qubit. Finally, the electric dipole terms implicit in the high frequency coupling enable strong coupling with superconducting microwave resonators, leading to more robust two-qubit gates.

We present a modulated microwave approach for quantum computing with qubits comprising three spins in a triple quantumdot. This approach includes single- and two-qubit gates that are protected against low-frequency electrical noise, due to an operating point with a narrowband response to high frequency electric fields. Furthermore, existing double quantumdot advances, including robust preparation and measurement via spin-to-charge conversion, are immediately applicable to the new qubit. Finally, the electric dipole terms implicit in the high frequency coupling enable strong coupling with superconducting microwave resonators, leading to more robust two-qubit gates. PMID:23952376

Colloidal quantumdot photovoltaics combine low-cost solution processing with quantum size-effect tuning to match absorption to the solar spectrum. Rapid advances have led to certified solar power conversion efficiencies of over 7%. Nevertheless, these devices remain held back by a compromise in the choice of quantumdot film thickness, balancing on the one hand the need to maximize photon absorption, mandating a thicker film, and, on the other, the need for efficient carrier extraction, a consideration that limits film thickness. Here we report an architecture that breaks this compromise by folding the path of light propagating in the colloidal quantumdot solid. Using this method, we achieve a substantial increase in short-circuit current, ultimately leading to improved power conversion efficiency. PMID:23835564

We review our recent advances in four-wave mixing spectroscopy of single semiconductor quantumdots using heterodyne spectral interferometry, a novel implementation of transient nonlinear spectroscopy allowing the study of the transient nonlinear polarization emitted from individual electronic transitions in both amplitude and phase. We present experiments on individual excitonic transitions localized in monolayer islands of GaAs/AlAs quantum wells and in self-assembled CdTe/ZnTe quantumdots. We investigate the formation of the photon echo from individual transitions, both with increasing number of transitions in the ensemble, and in the presence of temporal jitter of the energy of a single transition. The detection of amplitude and phase of the signal allows the implementation of a two-dimensional femtosecond spectroscopy, in which mutual coherent coupling of single quantumdot states can observed and quantified. PMID:21483055

Graphene nanoribbon quantumdot qubits have been proposed as promising candidates for quantum computing applications to overcome the spin-decoherence problems associated with typical semiconductor (e.g., GaAs) quantumdot qubits. We perform theoretical studies of the electronic structures of graphene nanoribbon quantumdots by solving the Dirac equation with appropriate boundary conditions. We then evaluate the exchange splitting based on an unrestricted Hartree-Fock method for the Dirac particles. The electronic wave function and long-range exchange coupling due to the Klein tunneling and the Coulomb interaction are calculated for various gate configurations. It is found that the exchange coupling between qubits can be significantly enhanced by the Klein tunneling effect. The implications of our results for practical qubit construction and operation are discussed. This work was supported in part by the Ministry of Science and Technology, Taiwan, under Contract No. MOST 104-2112-M-001-009-MY2.

Tunneling in a quantum coherent structure is not restricted to only nearest neighbors. Hopping between distant sites is possible via the virtual occupation of otherwise avoided intermediate states. Here we report the observation of long-range transitions in the transport through three quantumdots coupled in series. A single electron is delocalized between the left and right quantumdots, while the center one remains always empty. Superpositions are formed, and both charge and spin are exchanged between the outermost dots. The delocalized electron acts as a quantum bus transferring the spin state from one end to the other. Spin selection is enabled by spin correlations. The process is detected via the observation of narrow resonances which are insensitive to Pauli spin blockade.

Tunneling in a quantum coherent structure is not restricted to only nearest neighbors. Hopping between distant sites is possible via the virtual occupation of otherwise avoided intermediate states. Here we report the observation of long-range transitions in the transport through three quantumdots coupled in series. A single electron is delocalized between the left and right quantumdots, while the center one remains always empty. Superpositions are formed, and both charge and spin are exchanged between the outermost dots. The delocalized electron acts as a quantum bus transferring the spin state from one end to the other. Spin selection is enabled by spin correlations. The process is detected via the observation of narrow resonances which are insensitive to Pauli spin blockade. PMID:24836266

We report on the observation of periodic conductance oscillations near quantum Hall plateaus in suspended graphene nanoribbons. They are attributed to single quantumdots that are formed in the narrowest part of the ribbon, in the valleys and hills of a disorder potential. In a wide flake with two gates, a double-dot system's signature has been observed. Electrostatic confinement is enabled in single-layer graphene due to the gaps that are formed between the Landau levels, suggesting a way to create gate-defined quantumdots that can be accessed with quantum Hall edge states. PMID:27198562

CdSe quantumdot has great potential in various applications especially for emitting devices. One example potential application of CdSe quantumdot is security label for anti-counterfeiting. In this work, we present a practical approach of security label on paper using one and two colors of colloidal CdSe quantumdot, which is used as stamping ink on various types of paper. Under ambient condition, quantumdot is almost invisible. The quantumdot security label can be revealed by detecting emission of quantumdot using photoluminescence and cnc machine. The recorded quantumdot emission intensity is then analyzed using home-made program to reveal quantumdot pattern stamp having the word 'RAHASIA'. We found that security label using quantumdot works well on several types of paper. The quantumdot patterns can survive several days and further treatment is required to protect the quantumdot. Oxidation of quantumdot that occurred during this experiment reduced the emission intensity of quantumdot patterns.

The role of nuclear spin fluctuations in the dynamic polarization of nuclear spins by electrons is investigated in (In,Ga)As/GaAs quantumdots. The photoluminescence polarization under circularly polarized optical pumping in transverse magnetic fields (Hanle effect) is studied. A weak additional magnetic field parallel to the optical axis is used to control the efficiency of nuclear spin cooling and the sign of nuclear spin temperature. The shape of the Hanle curve is drastically modified when changing this control field, as observed earlier in bulk semiconductors and quantum wells. However, the standard nuclear spin cooling theory, operating with the mean nuclear magnetic field (Overhauser field), fails to describe the experimental Hanle curves in a certain range of control fields. This controversy is resolved by taking into account the nuclear spin fluctuations owed to the finite number of nuclei in the quantumdot. We propose a model considering cooling of the nuclear spin system by electron spins experiencing fast vector precession in the random Overhauser fields of nuclear spin fluctuations. The model allows us to accurately describe the measured Hanle curves and to evaluate the parameters of the electron-nuclear spin system of the studied quantumdots.

Multiple exciton generation in quantumdots (QDs) has been intensively studied as a way to enhance solar energy conversion by channeling the excess photon energy (energy greater than the bandgap) to produce multiple electron-hole pairs. Among other useful properties, quantum confinement can both increase Coulomb interactions that drive the MEG process and decrease the electron-phonon coupling that cools hot-excitons in bulk semiconductors. We have demonstrated that MEG in PbSe QDs is about two times as efficient at producing multiple electron-hole pairs than bulk PbSe. I will discuss our recent results investigating MEG in PbSe, PbS and PbSxSe1-x, which exhibits an interesting size-dependence of the MEG efficiency. Thin films of electronically coupled PbSe QDs have shown promise in simple photon-to-electron conversion architectures with power conversion efficiencies above 5%. We recently reported an enhancement in the photocurrent resulting from MEG in PbSe QD-based solar cells. We find that the external quantum efficiency (spectrally resolved ratio of collected charge carriers to incident photons) peaked at 114% in the best devices measured, with an internal quantum efficiency of 130%. These results demonstrate that MEG charge carriers can be collected in suitably designed QD solar cells. We compare our results to transient absorption measurements and find reasonable agreement.

Single spins in semiconductor quantumdots form a promising platform for solid-state quantum information processing. The spin-up and spin-down states of a single electron or hole, trapped inside a quantumdot, can represent a single qubit with a reasonably long decoherence time. The spin qubit can be optically coupled to excited (charged exciton) states that are also trapped in the quantumdot, which provides a mechanism to quickly initialize, manipulate and measure the spin state with optical pulses, and to interface between a stationary matter qubit and a 'flying' photonic qubit for quantum communication and distributed quantum information processing. The interaction of the spin qubit with light may be enhanced by placing the quantumdot inside a monolithic microcavity. An entire system, consisting of a two-dimensional array of quantumdots and a planar microcavity, may plausibly be constructed by modern semiconductor nano-fabrication technology and could offer a path toward chip-sized scalable quantum repeaters and quantum computers. This article reviews the recent experimental developments in optical control of single quantumdot spins for quantum information processing. We highlight demonstrations of a complete set of all-optical single-qubit operations on a single quantumdot spin: initialization, an arbitrary SU(2) gate, and measurement. We review the decoherence and dephasing mechanisms due to hyperfine interaction with the nuclear-spin bath, and show how the single-qubit operations can be combined to perform spin echo sequences that extend the qubit decoherence from a few nanoseconds to several microseconds, more than 5 orders of magnitude longer than the single-qubit gate time. Two-qubit coupling is discussed, both within a single chip by means of exchange coupling of nearby spins and optically induced geometric phases, as well as over longer-distances. Long-distance spin-spin entanglement can be generated if each spin can emit a photon that is entangled

Single spins in semiconductor quantumdots form a promising platform for solid-state quantum information processing. The spin-up and spin-down states of a single electron or hole, trapped inside a quantumdot, can represent a single qubit with a reasonably long decoherence time. The spin qubit can be optically coupled to excited (charged exciton) states that are also trapped in the quantumdot, which provides a mechanism to quickly initialize, manipulate and measure the spin state with optical pulses, and to interface between a stationary matter qubit and a ‘flying’ photonic qubit for quantum communication and distributed quantum information processing. The interaction of the spin qubit with light may be enhanced by placing the quantumdot inside a monolithic microcavity. An entire system, consisting of a two-dimensional array of quantumdots and a planar microcavity, may plausibly be constructed by modern semiconductor nano-fabrication technology and could offer a path toward chip-sized scalable quantum repeaters and quantum computers. This article reviews the recent experimental developments in optical control of single quantumdot spins for quantum information processing. We highlight demonstrations of a complete set of all-optical single-qubit operations on a single quantumdot spin: initialization, an arbitrary SU(2) gate, and measurement. We review the decoherence and dephasing mechanisms due to hyperfine interaction with the nuclear-spin bath, and show how the single-qubit operations can be combined to perform spin echo sequences that extend the qubit decoherence from a few nanoseconds to several microseconds, more than 5 orders of magnitude longer than the single-qubit gate time. Two-qubit coupling is discussed, both within a single chip by means of exchange coupling of nearby spins and optically induced geometric phases, as well as over longer-distances. Long-distance spin-spin entanglement can be generated if each spin can emit a photon that is

Submonolayer quantumdots as active medium in opto-electronic devices promise to combine the high density of states of quantum wells with the fast recovery dynamics of self-assembled quantumdots. We investigate the gain and phase recovery dynamics of a semiconductor optical amplifier based on InAs submonolayer quantumdots in the regime of linear operation by one- and two-color heterodyne pump-probe spectroscopy. We find an as fast recovery dynamics as for quantumdot-in-a-well structures, reaching 2 ps at moderate injection currents. The effective quantum well embedding the submonolayer quantumdots acts as a fast and efficient carrier reservoir.

Semiconductor quantumdots have paid much attention as it is a promising candidate for quantum, optical devices, such as quantum computer and quantumdot laser. We propose a local emission energy control method of semiconductor quantumdots using applying strain by volume expansion of phase change material. Phase change material can change its phase crystalline to amorphous, and the volume expand by its phase change. This method can control energy shift direction and amount by amorphous religion and depth. Using this method, we matched emission energy of two InAs/InP quantumdots. This achievement can connect to observing superradiance phenomenon and quantumdot coupling effect.

This is the final report of a three-year, Laboratory-Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). Research was performed in analytic and computational techniques for dealing with hard chaos, especially the powerful tool of cycle expansions. This work has direct application to the understanding of electrons in nanodevices, such as junctions of quantum wires, or in arrays of dots or antidots. We developed a series of techniques for computing the properties of quantum systems with hard chaos, in particular the flow of electrons through nanodevices. These techniques are providing the insight and tools to design computers with nanoscale components. Recent efforts concentrated on understanding the effects of noise and orbit pruning in chaotic dynamical systems. We showed that most complicated chaotic systems (not just those equivalent to a finite shift) will develop branch points in their cycle expansion. Once the singularity is known to exist, it can be removed with a dramatic increase in the speed of convergence of quantities of physical interest.

We investigate the absorptive-dispersive properties of a weak probe field in a ladder-type quantumdot. It is found that the enhanced refraction index without absorption can be easily controlled via adjusting properly the corresponding parameters of the system. Our scheme may provide some new possibilities for technological applications in dispersion compensation and solid-state quantum communication for quantum information processing.

Fluorescence is a tool widely employed in biological assays. Fluorescent semiconducting nanocrystals, quantumdots (QDs), are beginning to find their way into the tool box of many biologist, chemist and biochemist. These quantumdots are an attractive alternative to the traditional organic dyes due to their broad excitation spectra, narrow emission spectra and photostability. Quantumdots were used to detect and monitor the progession of viral glycoproteins, F (fusion) and G (attachment), from Respiratory Syncytial Virus (RSV) in HEp-2 cells. Additionally, oligo-Qdot RNA probes have been developed for identification and detection of mRNA of the N(nucleocapsid) protein for RSV. The use of quantumdot-FISH probes provides another confirmatory route to diagnostics as well as a new class of probes for monitoring the flux and fate of viral RNA RSV is the most common cause of lower respiratory tract infection in children worldwide and the most common cause of hospitalization of infants in the US. Antiviral therapy is available for treatment of RSV but is only effective if given within the first 48 hours of infection. Existing test methods require a virus level of at least 1000-fold of the amount needed for infection of most children and require several days to weeks to obtain results. The use of quantumdots may provide an early, rapid method for detection and provide insight into the trafficking of viral proteins during the course of infection.

Quantumdots are semiconductor nanoparticles that have gained popularity in optical and electronic applications in recent years. Aptamers are short man-made oligonucleotides with high binding affinity for a specific target. One part of this work presents an optical FRET-based sensor for K+ and Pb2+ consisting of a fluorescent quantumdot, an aptamer, and a gold nanoparticle quencher. Additionally, an electrochemical sensor for K+ and Pb2+ is also presented, which consists of an aptamer with an electron donor bound to graphene. Both sensors are shown to detect K+ and Pb2+ at concentrations critical for human health. The emission spectrum of the optical sensor is also shown to shift in response to strong electric fields. UV-excited TiO 2 quantumdots are also investigated for their ability to influence the dynamics of voltage gated ion channels in cells. It was found that the activation voltage is shifted in the presence of UV-excited TiO2 quantumdots. Electrostatic force measurements and theoretical calculations confirm that electric fields in TiO2 can in fact be optically induced. ZnO quantumdots are also synthesized and their optical and electrical properties are similarly investigated. Additionally, Raman and surface-enhanced Raman spectroscopy is used in this work to find previously-unknown spectra of the aptamer Apt-alphavbeta3 and the peptide thymosin-beta4.

We study optical and magnetic properties of CdSe and Cd-Mn-Se quantumdots (QD). We find that there are two luminescence lines, one is fast and another is slow (~1ms). With the increase of a QD diameter the slow luminescence disappears at some critical QD size, thus only one line (fast) remains. Using the SAC SI computational method we find that D = 3.2 nm and D = 2.7 nm if the Mn impurity is located inside a QD or on a QD surface, respectively. For two or four Mn atoms in the quantumdot, now absorption takes place because the transition is spin-allowed. The DFT calculations of the magnetic state reveal that it is an antiferromagnet. We also study other quantumdots such as Cd-Mn-Se, Zn-Mn-S, and Zn-Mn-Se, doped and undoped. We find the slow luminescence energies for low concentrations of Mn impurities for each QD type. The calculations indicate that two luminescence lines, fast and slow, should always take place. However for Pb-Mn-S quantumdots there are now Mn levels inside a HOMO-LUMO gap, i.e., the Mn-levels are located in a PbS conduction band. The presence of Mn dopants increases the band gap and also removes the exciton peak. This effect is different to the other quantumdots.

Colloidal semiconductor quantumdots are fluorescent nanocrystals exhibiting exceptional optical properties, but their emission intensity strongly depends on their charging state and local environment. This leads to blinking at the single-particle level or even complete fluorescence quenching, and limits the applications of quantumdots as fluorescent particles. Here, we show that a single quantumdot encapsulated in a silica shell coated with a continuous gold nanoshell provides a system with a stable and Poissonian emission at room temperature that is preserved regardless of drastic changes in the local environment. This novel hybrid quantumdot/silica/gold structure behaves as a plasmonic resonator with a strong Purcell factor, in very good agreement with simulations. The gold nanoshell also acts as a shield that protects the quantumdot fluorescence and enhances its resistance to high-power photoexcitation or high-energy electron beams. This plasmonic fluorescent resonator opens the way to a new family of plasmonic nanoemitters with robust optical properties. PMID:25581887

A group of quantumdots can be designed to have a unique spectral emission by varying the size of the quantumdots (wavelength) and number of quantumdots (intensity). This technique has been previously proposed for biological tags and object identification. The potential of this system lies in the ability to have a large number of distinguishable wavelengths and intensity levels. This paper presents a communications system model for MxQDs including the interference between neighbouring QD colours and detector noise. An analytical model of the signal-to-noise ratio of a Charge-Coupled Device (CCD) spectrometer is presented and confirmed with experimental results. We then apply a communications system perspective and propose data detection algorithms that increase the readability of the quantumdots tags. It is demonstrated that multiplexed quantumdot barcodes can be read with 99.7% accuracy using the proposed data detection algorithms in a system with 6 colours and 6 intensity values resulting in 46,655 unique spectral codes. PMID:22418382

In this dissertation, I present photonics applications of PbS quantum-dot-doped (QD-doped) glasses. The dissertation consists of two major parts: bulk material applications (Cr:forsterite laser modelocking, bleaching dynamics, optical gain, and photo-luminescence) and the fabrication of QD-doped ion-exchanged waveguides. When this work began, these PbS QD-doped glasses were the state-of-the-art in quantumdot glasses due to their narrow size distribution. Modelocking of a Cr:forsterite laser using this glass as a saturable absorber had been demonstrated, with little understanding of the dynamics. This work began by studying the dynamics of the saturable absorber to explain the ps-pulse width. In the bulk measurements, I functioned as a secondary researcher. In the laser modelocking and bleaching measurements, my contribution was laser cavity alignment, sample preparation, collecting autocorrelation traces, and aiding in the setup and data collection for the bleaching measurements. On this work, I coauthored one refereed journal article in Applied Physics Letters [1] and one refereed conference paper [2], for which I am third and second author, respectively. For the gain measurements, I aided in the setup and data collection, whereas I setup and took most of the luminescence data. The gain measurements resulted in one second-author refereed journal article in Applied Physics Letters [3] and I presented the luminescence results at CLEO2000 [4]. I took the lead role in the waveguide fabrication and characterization and authored refereed journal articles in Applied Physics Letters [5], Journal of Applied Physics [6], and Journal of the Optical Society of America B [7]. I also presented an invited talk at Photonics West [8] and presented at CLEO200-1 [9]. Additionally, I have been a coauthor of presentations at the Nanotechnology Symposium (2006), American Ceramic Society [10], and Photonics Europe ( 2006) [11]. A book chapter in The Photonics Handbook, 2nd edition [12

A plurality of quantumdots each have a shell. The quantumdots are embedded in an organic matrix. At least the quantumdots and the organic matrix are photoconductive semiconductors. The shell of each quantumdot is arranged as a tunneling barrier to require a charge carrier (an electron or a hole) at a base of the tunneling barrier in the organic matrix to perform quantum mechanical tunneling to reach the respective quantumdot. A first quantum state in each quantumdot is between a lowest unoccupied molecular orbital (LUMO) and a highest occupied molecular orbital (HOMO) of the organic matrix. Wave functions of the first quantum state of the plurality of quantumdots may overlap to form an intermediate band.

Entanglement plays a central role in fundamental tests of quantum mechanics as well as in the burgeoning field of quantum information processing. Particularly in the context of quantum networks and communication, some of the major challenges are the efficient generation of entanglement between stationary (spin) and propagating (photon) qubits, the transfer of information from flying to stationary qubits, and the efficient generation of entanglement between distant stationary (spin) qubits. In this talk, I will present such experimental implementations achieved in our team with semiconductor self-assembled quantumdots.Not only are self-assembled quantumdots good single-photon emitters, but they can host an electron or a hole whose spin serves as a quantum memory, and then present spin-dependent optical selection rules leading to an efficient spin-photon quantum interface. Moreover InGaAs quantumdots grown on GaAs substrate can profit from the maturity of III-V semiconductor technology and can be embedded in semiconductor structures like photonic cavities and Schottky diodes.I will report on the realization of heralded quantum entanglement between two semiconductor quantumdot hole spins separated by more than five meters. The entanglement generation scheme relies on single photon interference of Raman scattered light from both dots. A single photon detection projects the system into a maximally entangled state. We developed a delayed two-photon interference scheme that allows for efficient verification of quantum correlations. Moreover the efficient spin-photon interface provided by self-assembled quantumdots allows us to reach an unprecedented rate of 2300 entangled spin pairs per second, which represents an improvement of four orders of magnitude as compared to prior experiments carried out in other systems.Our results extend previous demonstrations in single trapped ions or neutral atoms, in atom ensembles and nitrogen vacancy centers to the domain of

Optical properties and growth of self-assembled quantumdots (SAQDs) for optoelectronic device applications are discussed. After briefly reviewing the history of research on QD lasers, we discuss growth of InAs SQDs including the light emission at the wavelength of 1.52)mum with a narrow linewidth (22 meV) and the area-controlled growth which demonstrates formation of SAQDs in selected local areas on a growth plane using a SiO)-2) mask with MOCVD growth. Then properties of the InGaAs AQDs are investigated by the near- field photoluminescence excitation spectroscopy which reveals gradually increasing continuum absorption connected with the two-dimensional-like (2D-like) wetting layer, resulting in faster relaxation of electrons due to a crossover between OD and 2D character in the density of states. Moreover, we have investigated InGaN self-assembled QDs on a GaN layer achieving the average diameter as small as 8.4nm and a strong light at room temperature. A laser structure with the stacked InGAN QDs embedded in the active layer was fabricated and room temperature operation of blue InGaN QD lasers was achieved under optical excitation. Carrier confinement in QDs was examined using near-field $DAL- photoluminescence measurement: A very sharp spectral line emitted from excitons in individual InGaN QDs was observed. Establishing AlGaN/GaN DBR of high quality, we succeeded in lasing action in InGaN blue light emitting VCSELs. Enhancement of spontaneous emission is demonstrated. Finally, perspective of QD lasers.

Semiconductor quantumdots (QDs) have served as the basis for signal development in a variety of biosensing technologies and in applications using bioprobes. The use of QDs as physical platforms to develop biosensors and bioprobes has attracted considerable interest. This is largely due to the unique optical properties of QDs that make them excellent choices as donors in fluorescence resonance energy transfer (FRET) and well suited for optical multiplexing. The large majority of QD-based bioprobe and biosensing technologies that have been described operate in bulk solution environments, where selective binding events at the surface of QDs are often associated with relatively long periods to reach a steady-state signal. An alternative approach to the design of biosensor architectures may be provided by a microfluidic system (MFS). A MFS is able to integrate chemical and biological processes into a single platform and allows for manipulation of flow conditions to achieve, by sample transport and mixing, reaction rates that are not entirely diffusion controlled. Integrating assays in a MFS provides numerous additional advantages, which include the use of very small amounts of reagents and samples, possible sample processing before detection, ultra-high sensitivity, high throughput, short analysis time, and in situ monitoring. Herein, a comprehensive review is provided that addresses the key concepts and applications of QD-based microfluidic biosensors with an added emphasis on how this combination of technologies provides for innovations in bioassay designs. Examples from the literature are used to highlight the many advantages of biosensing in a MFS and illustrate the versatility that such a platform offers in the design strategy. PMID:22163723

Once released into the environment, engineered nanoparticles (eNPs) are subjected to processes that may alter their physical or chemical properties, potentially altering their toxicity vis-à-vis the as-synthesized materials. We examined the toxicity to zebrafish embryos of CdSecore/ZnSshell quantumdots (QDs) before and after exposure to an in vitro chemical model designed to simulate oxidative weathering in soil environments based on a reductant-driven Fenton’s reaction. Exposure to these oxidative conditions resulted in severe degradation of the QDs: the Zn shell eroded, Cd2+ and selenium were released, and amorphous Se-containing aggregates were formed. Weathered QDs exhibited higher potency than did as-synthesized QDs. Morphological endpoints of toxicity included pericardial, ocular and yolk sac edema, non-depleted yolk, spinal curvature, tail malformations, and craniofacial malformations. To better understand the selenium-like toxicity observed in QD exposures, we examined the toxicity of selenite, selenate and amorphous selenium nanoparticles (SeNPs). Selenite exposures resulted in high mortality to embryos/larvae while selenate and SeNPs were non-toxic. Co-exposures to SeNPs + CdCl2 resulted in dramatic increase in mortality and recapitulated the morphological endpoints of toxicity observed with weathered QD exposures. Cadmium body burden was increased in larvae exposed to weathered QDs or SeNP + CdCl2 suggesting the increased potency of weathered QDs was due to selenium modulation of cadmium toxicity. Our findings highlight the need to examine the toxicity of eNPs after they have undergone environmental weathering processes. PMID:23815598

Luminescent silicon quantumdots (SiQDs) are emerging as attractive materials for optoelectronic devices, third generation photovoltaics, and bioimaging. Their applicability in the real world is contingent on their optical properties and long-term environmental stability; and in biological applications, factors such as water solubility and toxicity must also be taken into consideration. The aforementioned properties are highly dependent on the QDs' surface chemistry. In this work, SiQDs were engineered for the respective applications using liquid-phase and gas-phase functionalization techniques. Preliminary work in luminescent downshifting for photovoltaic systems are also reported. Highly luminescent SiQDs were fabricated by grafting unsaturated hydrocarbons onto the surface of hydrogen-terminated SiQDs via thermal and photochemical hydrosilylation. An industrially attractive, all gas-phase, nonthermal plasma synthesis, passivation (aided by photochemical reactions), and deposition process was also developed to reduce solvent waste. With photoluminescence quantum yields (PLQYs) nearing 60 %, the alkyl-terminated QDs are attractive materials for optical applications. The functionalized SiQDs also exhibited enhanced thermal stability as compared to their unfunctionalized counterparts, and the photochemically-hydrosilylated QDs further displayed photostability under UV irradiation. These environmentally-stable SiQDs were used as luminescent downshifting layers in photovoltaic systems, which led to enhancements in the blue photoresponse of heterojunction solar cells. Furthermore, the QD films demonstrated antireflective properties, improving the coupling efficiency of sunlight into the cell. For biological applications, oxide, amine, or hydroxyl groups were grafted onto the surface to create water-soluble SiQDs. Luminescent, water-soluble SiQDs were produced in by microplasma treating the QDs in water. Stable QYs exceeding 50 % were obtained. Radical-based and

In this thesis, we study charge transport in arrays of semiconductor nanocrystal quantumdots. Nanocrystals are synthesized in solution, and an organic ligand on the surface of the nanocrystal creates a potential barrier that confines charges in the nanocrystal. Optical absorption measurements reveal discrete electronic energy levels in the nanocrystals resulting from quantum confinement. When nanocrystals are deposited on a surface, they self-assemble into a close-packed array forming a nanocrystal solid. We report electrical transport measurements of a PbSe nanocrystal solid that serves as the channel of an inverted field-effect transistor. We measure the conductance as a function of temperature, source-drain bias and. gate voltage. The data indicates that holes are the majority carriers; the Fermi energy lies in impurity states in the bandgap of the nanocrystal; and charges hop between the highest occupied valence state in the nanocrystals (the 1S h states). At low source-drain voltages, the activation energy for hopping is given by the energy required to generate holes in the 1Sh state plus activation over barriers resulting from site disorder. The barriers from site disorder are eliminated with a sufficiently high source-drain bias. From the gate effect, we extract the Thomas-Fermi screening length and a density of states that is consistent with the estimated value. We consider variable-range hopping as an alternative model, and find no self-consistent evidence for it. Next, we employ charge sensing as an alternative to current measurements for studying transport in materials with localized sites. A narrow-channel MOSFET serves as a charge sensor because its conductance is sensitive to potential fluctuations in the nearby environment caused by the motion of charge. In particular, it is sensitive to the fluctuation of single electrons at the silicon-oxide interface within the MOSFET. We pattern a strip of amorphous germanium within 100 nm of the transistor. The

The practical applicability of a semiconductor quantumdot with spin-orbit interaction gives an impetus to study analytical solutions to one- and two-electron quantumdots with or without a magnetic field.

Carbon dots, generally defined as small carbon nanoparticles with various surface passivation schemes, have emerged as a new class of quantum-dot-like nanomaterials, with their optical properties and photocatalytic functions resembling those typically found in conventional nanoscale semiconductors. In this work, carbon dots were evaluated for their photoinduced bactericidal functions, with the results suggesting that the dots were highly effective in bacteria-killing with visible-light illumination. In fact, the inhibition effect could be observed even simply under ambient room lighting conditions. Mechanistic implications of the results are discussed and so are opportunities in the further development of carbon dots into a new class of effective visible/natural light-responsible bactericidal agents for a variety of bacteria control applications. PMID:27064729

We investigate the bright-to-dark exciton conversion efficiency in type-II quantumdots subject to a perpendicular magnetic field. To this end, we take the exciton storage protocol recently proposed by Simonin and co-workers [Phys. Rev. B 89, 075304 (2014)] and simulate its coherent dynamics. We confirm the storage is efficient in perfectly circular structures subject to weak external electric fields, where adiabatic evolution is dominant. In practice, however, the efficiency rapidly degrades with symmetry lowering. Besides, the use of excited states is likely unfeasible owing to the fast decay rates. We then propose an adaptation of the protocol which does not suffer from these limitations.

We report a room temperature optically pumped QuantumDot-based Spin-Vertical-External-Cavity Surface-Emitting laser (QD Spin-VECSEL) operating at the telecom wavelength of 1.3 μm. The active medium was composed of 5 × 3 QD layers; each threefold group was positioned at an antinode of the standing wave of the optical field. Circularly polarized lasing in the QD-VECSEL under Continuous-Wave optical pumping has been realized with a threshold pump power of 11 mW. We further demonstrate at room temperature control of the QD-VECSEL output polarization ellipticity via the pump polarization.

Photovoltaics are limited in their power conversion efficiency (PCE) by very rapid relaxation of energetic carriers to the band edge. Therefore, photons from the visible and ultraviolet parts of the spectrum typically are not efficiently converted into electrical energy. One approach that can address this is multiple exciton generation (MEG), where a single photon of sufficient energy can generate multiple excited electron-hole pairs. This process has been shown to be more efficient in quantumdots than bulk semiconductors, but it has never been demonstrated in the photocurrent of a solar cell. In order to demonstrate that multiple exciton generation can address fundamental limits for conventional photovoltaics, I have developed prototype devices from colloidal PbS and PbSe quantumdot inks. I have characterized both the colloidal suspensions and films of quantumdots with the goal of understanding what properties determine the efficiency of the solar cell and of the MEG process. I have found surface chemistry effects on solar cells, photoluminescence, and MEG, and I have found some chemical treatments that lead to solar cells showing MEG. These devices show external quantum efficiency (EQE) greater than 100% for certain parts of the solar spectrum, and I extract internal quantum efficiency (IQE) consistent with previous measurements of colloidal suspensions of quantumdots. These findings are a small first step toward breaking the single junction Shockley-Queisser limit of present-day first and second generation solar cells, thus moving photovoltaic cells toward a new regime of efficiency.

Photophysical properties of complexes of semiconductor quantumdots with conventional photosensitizers for photodynamic therapy (tetrapyrroles) were investigated. A luminescent study of complexes in aqueous solutions was performed using spectral- and time-resolved luminescence spectroscopy. It was found that increasing the photosensitizer relative concentration in complexes resulted in sharp drop of the nonradiative energy transfer efficiency and the quantum yield of the photosensitizer photoluminescence. This fact indicates that additional channels of nonradiative energy dissipation may take place in the complexes. Using complexes of Al(OH)-sulphophthalocyanine with CdSe/ZnS quantumdots in the aqueous solution as an typical example, we have demonstrated that new channels of the energy dissipation may arise due to aggregation of the photosensitizer molecules upon formation of the complexes with quantumdots. We also demonstrated that use of methods of complex formation preventing aggregation of photosensitizers allows to conserve the high energy transfer efficiency and quantum yield of the acceptor photoluminescence in complexes in wide range of the photosensitizer concentrations. We believe that our study allows obtaining new information about the physical mechanisms of nonradiative energy transfer in quantumdots-tetrapyrrole complexes perspective for photodynamic therapy.

Using the spatial-temporal symmetry principle we developed recently, we propose an effective scheme for even-harmonics generation in coupled quantumdots. The relative intensity of odd and even harmonic components in the emission spectrum can be controlled by tuning the dipole couplings among the dots, which can be realized in experiments by careful design of the nanostructures. In particular, pure 2nth harmonics and (2n+1)th harmonics (where n is an integer) can be generated simultaneously with polarizations in two mutual perpendicular directions in our systems. An experimental design of the coupled dots system is presented.

We consider the single electron confinement states in the system of two and three quantumdots (QDs). The InAs/GaAs QDs are modeled as laterally distributed dots, using single sub-band effective mass approach with effective potential simulating the strain effect. Electron localization in double quantumdots (DQDs) and in triple quantumdots (TQDs) is studied over the entire electron energy spectrum by varying the geometry parameters of these QDs arrays. It is shown that a small violation of the DQD shape symmetry drastically affects tunneling. This effect also appears as a numerical instability in calculations of spectral distribution of localized/delocalized electron states for small variations of the input parameters of numerical procedure. The effect of adding a third dot to a DQD is investigated. We show that the presence of a third dot increases the tunneling in the initial DQD. The spectral distribution of localized/delocalized states appears sensitive to the violation of the mirror symmetry of TQDs. This work was supported by the NSF (HRD-1345219).

We demonstrate multicolor three-dimensional super-resolution imaging with quantumdots (QSTORM). By combining quantumdot asynchronous spectral blueing with stochastic optical reconstruction microscopy and adaptive optics, we achieve three-dimensional imaging with 24 nm lateral and 37 nm axial resolution. By pairing two short-pass filters with two appropriate quantumdots, we are able to image single blueing quantumdots on two channels simultaneously, enabling multicolor imaging with high photon counts. PMID:25703291

Fluorescence resonance energy transfer in complexes of semiconductor CdSe/ZnS quantumdots with molecules of heterocyclic azo dyes, 1-(2-pyridylazo)-2-naphthol and 4-(2-pyridylazo) resorcinol, formed at high quantumdot concentration in the polymer pore track membranes were studied by steady-state and transient PL spectroscopy. The effect of interaction between the complexes and free quantumdots on the efficiency of the fluorescence energy transfer and quantumdot luminescence quenching was found and discussed.

Epitaxial attachment of quantumdots into ordered superlattices enables the synthesis of quasi-two-dimensional materials that theoretically exhibit features such as Dirac cones and topological states, and have major potential for unprecedented optoelectronic devices. Initial studies found that disorder in these structures causes localization of electrons within a few lattice constants, and highlight the critical need for precise structural characterization and systematic assessment of the effects of disorder on transport. Here we fabricated superlattices with the quantumdots registered to within a single atomic bond length (limited by the polydispersity of the quantumdot building blocks), but missing a fraction (20%) of the epitaxial connections. Calculations of the electronic structure including the measured disorder account for the electron localization inferred from transport measurements. The calculations also show that improvement of the epitaxial connections will lead to completely delocalized electrons and may enable the observation of the remarkable properties predicted for these materials.

Solution-processed photovoltaic technologies represent a promising way to reduce the cost and increase the efficiency of solar energy harvesting. Among these, colloidal semiconductor quantumdot photovoltaics have the advantage of a spectrally tuneable infrared bandgap, which enables use in multi-junction cells, as well as the benefit of generating and harvesting multiple charge carrier pairs per absorbed photon. Here we review recent progress in colloidal quantumdot photovoltaics, focusing on three fronts. First, we examine strategies to manage the abundant surfaces of quantumdots, strategies that have led to progress in the removal of electronic trap states. Second, we consider new device architectures that have improved device performance to certified efficiencies of 10.6%. Third, we focus on progress in solution-phase chemical processing, such as spray-coating and centrifugal casting, which has led to the demonstration of manufacturing-ready process technologies.

The use of semiconductor luminescent quantumdots for the labeling of biomolecules is rapidly expanding, however it still requires facile methods to attach functional globular proteins to biologically optimized quantumdots. Here we discuss the development of controlled variable length peptidyl linkers to attach biomolecules to poly(ethylene) glycol (PEG) coated quantumdots for both in vitro and in vivo applications. The peptides chosen, β-sheets and alpha helices are appended to polyhistidine sequences and this allows for control of the ratio of peptide bioconjugated to QD and the distance from QD to the biomolecule. Recombinant DNA engineering, bacterial peptide expression and Ni-NTA purification of histidine labeled peptides are utilized to create the linkers. Peptide length is confirmed by in vitro fluorescent resonance energy transfer (FRET).

We report Coulomb mediated hybridization of excitonic states in optically active InGaAs quantumdot molecules. By probing the optical response of an individual quantumdot molecule as a function of the static electric field applied along the molecular axis, we observe unexpected avoided level crossings that do not arise from the dominant single-particle tunnel coupling. We identify a new few-particle coupling mechanism stemming from Coulomb interactions between different neutral exciton states. Such Coulomb resonances hybridize the exciton wave function over four different electron and hole single-particle orbitals. Comparisons of experimental observations with microscopic eight-band k .p calculations taking into account a realistic quantumdot geometry show good agreement and reveal that the Coulomb resonances arise from broken symmetry in the artificial semiconductor molecule.

We report the observation of dressed states of a quantumdot. The optically excited exciton and biexciton states of the quantumdot are coupled by a strong laser field and the resulting spectral signatures are measured using differential transmission of a probe field. We demonstrate that the anisotropic electron-hole exchange interaction induced splitting between the x- and y-polarized excitonic states can be completely erased by using the ac-Stark effect induced by the coupling field, without causing any appreciable broadening of the spectral lines. We also show that by varying the polarization and strength of a resonant coupling field, we can effectively change the polarization axis of the quantumdot.

We study the properties of a quantumdot coupled to a normal lead and a time-reversal topological superconductor with Majorana Kramers pair at the end. We explore the phase diagram of the system as a function of Kondo and Majorana-induced coupling strengths using perturbative renormalization group study and slave-boson mean-field theory. We find that, in the presence of coupling between a quantumdot and a Majorana doublet, the system flows to a new fixed point controlled by the Majorana doublet, rather than the Kondo coupling, which is characterized by correlations between a localized spin and the fermion parity of each spin sector of the topological superconductor. We find that this fixed point is stable with respect to Gaussian fluctuations. We also investigate the effect of spin-spin interaction between a quantumdot and Majorana doublet and compare the result with a case where a normal lead is directly coupled to Majorana doublet.

: The coupling between DNA molecules and quantumdots can result in impressive nonlinear optical properties. In this paper, we theoretically demonstrate the significant enhancement of Kerr coefficient of signal light using optical pump-probe technique when the pump-exciton detuning is zero, and the probe-exciton detuning is adjusted properly to the frequency of DNA vibration mode. The magnitude of optical Kerr coefficient can be tuned by modifying the intensity of the pump beam. It is shown clearly that this phenomenon cannot occur without the DNA-quantumdot coupling. The present research will lead us to know more about the anomalous nonlinear optical behaviors in the hybrid DNA-quantumdot systems, which may have potential applications in the fields such as DNA detection. PMID:23194282

The authors report a two-color, colocated quantumdot based imaging system used to take multicolor images using a single focal plane array (FPA). The dots-in-a-well (DWELL) detectors consist of an active region composed of InAs quantumdots embedded in In.15Ga.85As quantum wells. DWELL samples were grown using molecular beam epitaxy and fabricated into 320×256 focal plane arrays with indium bumps. The FPA was then hybridized to an Indigo ISC9705 readout circuit and tested. Calibrated blackbody measurements at a device temperature of 77K yield midwave infrared and long wave infrared noise equivalent difference in temperature of ˜55 and 70mK.

We present a unique design and fabrication process for a lateral, gate-confined double quantumdot in an accumulation mode metal-oxide-semiconductor (MOS) structure coupled to an integrated microwave resonator. All electrostatic gates for the double quantumdot are contained in a single metal layer, and use of the MOS structure allows for control of the location of the two-dimensional electron gas via the location of the accumulation gates. Numerical simulations of the electrostatic confinement potential are performed along with an estimate of the coupling of the double quantumdot to the microwave resonator. Prototype devices are fabricated and characterized by transport measurements of electron confinement and reflectometry measurements of the microwave resonator.

A theoretical investigation is presented on the properties of charge transfer excitons at vertically coupled semiconductor quantumdots in the presence of electric and magnetic fields directed along the growth axis. Such excitons should have two interesting characteristics: an extremely long lifetime and a permanent dipole moment. We show that wave functions and the low-lying energies of charge transfer exciton can be found exactly for a special morphology of quantumdots that provides a parabolic confinement inside the layers. To take into account a difference between confinement potentials of an actual structure and of our exactly solvable model, we use the Galerkin method. The density of energy states is calculated for different InAs/GaAs quantumdots' dimensions, the separation between layers, and the strength of the electric and magnetic fields. A possibility of a formation of a giant dipolar momentum under external electric field is predicted. PMID:23092373

Graphical abstract: Display Omitted Highlights: ► A humidity sensing device was fabricated based on carbon quantumdots (CQDs) films. ► The conductivity of the CQDs films shows a linear and rapid response to atmosphere humidity. ► The humidity sensing property was due to the hydrogen bonds between the functional groups on CQDs. -- Abstract: We reported the fabrication of a humidity sensing device based on carbon quantumdots (CQDs) film. The conductivity of the CQDs film has a linear and rapid response to relative humidity, providing the opportunity for the fabrication of humidity sensing devices. The mechanism of our humidity sensor was proposed to be the formation of hydrogen bonds between carbon quantumdots and water molecules in the humidity environment, which significantly promote the electrons migration. In a control experiment, this hypothesis was confirmed by comparing the humidity sensitivity of candle soot (i.e. carbon nanoparticles) with and without oxygen containing groups on the surfaces.

The conduction band for electrons in layered Si nanostructures oriented along (001) has two low-lying valleys. Most theoretical treatments assume that these valleys are decoupled from the long-wavelength physics of electron confinement. In this work, we show that even a minimal amount of disorder (a single atomic step at the quantum well interface) is sufficient to mix valley states and electron orbitals, causing a significant distortion of the long-wavelength electron envelope. For physically realistic electric fields and dot sizes, this valley-orbit coupling impacts all electronic states in Si quantumdots, implying that one must always consider valley-orbit hybrid states, rather than distinct valley and orbital degrees of freedom. We discuss the ramifications of our results on silicon quantumdot qubits. This work was supported in part by ARO (W911NF-08-1-0482) and NSF (DMR-0805045).

A brief review will be given on coherent behaviour in serial triple quantumdots in AlGaAs/GaAs heterostructure related to multi-spin states. One series of experiments involves the application of coherent superpositions of multi-electron states to the transfer of single spins and two-spin states non-locally between edge quantumdots while maintaining the center quantumdot occupation fixed at one or zero electrons. A second series of experiments involves the identification of coherent leakage mechanisms away from targeted encoded three-spin states qubits. Finally, results will be shown which reveal an unexpected control of the gap at the S-T + anticrossing by taking advantage of different nuclear dynamic polarization pumping rates.

We present a theory of electronic properties of HgTe quantumdot and propose a strain sensor based on a strain-driven transition from a HgTe quantumdot with inverted bandstructure and robust topologically protected quantum edge states to a normal state without edge states in the energy gap. The presence or absence of edge states leads to large on/off ratio of conductivity across the quantumdot, tunable by adjusting the number of conduction channels in the source-drain voltage window. The electronic properties of a HgTe quantumdot as a function of size and applied strain are described using eight-band Luttinger and Bir-Pikus Hamiltonians, with surface states identified with chirality of Luttinger spinors and obtained through extensive numerical diagonalization of the Hamiltonian. PMID:24811674

We study the tunneling transport properties through a system of parallel quantumdots which are coupled to Majorana bound states (MBSs). The conductance and spectral function are computed using the retarded Green's function method based on the equation of motion. The conductance of the system is 2e{sup 2}/h at zero Fermi energy and is robust against the coupling between the MBSs and the quantumdots. The dependence of the Fermi energy on the spectral function is different for the first dot (dot1) than for the second dot (dot2) with fixed dot2-MBSs coupling. The influence of the Majorana bound states on the spectral function was studied for the series and parallel configurations of the system. It was found that when the configuration is in series, the Majorana bound states play an important role, resulting in a spectral function with three peaks. However, the spectral function shows two peaks when the system is in a parallel configuration. The zero Fermi energy spectral function is always 1/2 not only in series but also in the parallel configuration and robust against the coupling between the MBSs and the quantumdots. The phase diagram of the Fermi energy versus the quantumdot energy levels was also investigated.

We present the Quantum Computer Aided Design (QCAD) simulator that targets modeling quantum devices, particularly silicon double quantumdots (DQDs) developed for quantum qubits. The simulator has three di erentiating features: (i) its core contains nonlinear Poisson, e ective mass Schrodinger, and Con guration Interaction solvers that have massively parallel capability for high simulation throughput, and can be run individually or combined self-consistently for 1D/2D/3D quantum devices; (ii) the core solvers show superior convergence even at near-zero-Kelvin temperatures, which is critical for modeling quantum computing devices; (iii) it couples with an optimization engine Dakota that enables optimization of gate voltages in DQDs for multiple desired targets. The Poisson solver includes Maxwell- Boltzmann and Fermi-Dirac statistics, supports Dirichlet, Neumann, interface charge, and Robin boundary conditions, and includes the e ect of dopant incomplete ionization. The solver has shown robust nonlinear convergence even in the milli-Kelvin temperature range, and has been extensively used to quickly obtain the semiclassical electrostatic potential in DQD devices. The self-consistent Schrodinger-Poisson solver has achieved robust and monotonic convergence behavior for 1D/2D/3D quantum devices at very low temperatures by using a predictor-correct iteration scheme. The QCAD simulator enables the calculation of dot-to-gate capacitances, and comparison with experiment and between solvers. It is observed that computed capacitances are in the right ballpark when compared to experiment, and quantum con nement increases capacitance when the number of electrons is xed in a quantumdot. In addition, the coupling of QCAD with Dakota allows to rapidly identify which device layouts are more likely leading to few-electron quantumdots. Very efficient QCAD simulations on a large number of fabricated and proposed Si DQDs have made it possible to provide fast feedback for design

It is a fairly common occurrence that, in top-gated Si quantumdots, the dots appear in reproducible but unexpected positions. For instance, sometimes a group will make gates in order to electrostatically generate tunnel barriers, but discover that the quantumdot is formed underneath the gate rather than between two barrier gates. We will discuss the possibility that such quantumdots arise from the mechanical strain induced by the gate. The model is simple: i) We simulate metal or polysilicon gates on top of a Si/SiO2 wafer, and calculate the stress and strain from differential thermal contraction of the materials; ii) Using the fact that the energy of the Si conduction band depends on strain through the deformation potential, we then convert the strain modulation to a potential energy modulation. As an example, we find that, for a single Al gate, there is a potential well directly underneath the gate with the size of a few meV, in agreement with recent experimental results. We also show that polysilicon gates will not produce such strain-induced quantumdots.

We present the initial results of entraining colloidal quantumdots emitting at wavelengths from 0.5um through 1.2um, in various micro-structured optical fibers. Conventional and non-conventional, micro-structured optical fibers fabricated at Virginia Tech"s Fiber & ElectroOptics Research Center (FEORC) have been combined with semiconductor, colloidal quantumdots fabricated by the VT Advanced Biomedical Center (VTabc). The results are presented primarily in the form of visual verification and analysis of entrainment phenomena, for a cross-section of colloidal dot and micro-structured fiber forms. Unique optical, electro-optical and material properties resulting from the combinations are visibly suggested in the results. Core/clad/free space propagation properties and effects of emitted and absorbed light fields are observed to be dependent on the structure, aspect ratio and materials of the fibers as well as the properties of the colloidal quantumdots. Basic spectral data on representative free-space materials will be presented in the current paper. The presentation will explore in passing, the research options available to such quantumdot-fiber combinations, including advanced sensors, sources and filters.

We measure time correlators of a spin qubit in an optically active quantumdot beyond the second order. Such higher-order correlators are shown to be directly sensitive to pure quantum effects that cannot be explained within the classical framework. They allow direct determination of ensemble and quantum dephasing times, T2* and T2, using only repeated projective measurements and without the need for coherent spin control. Our method enables studies of purely quantum behavior in solid state systems, including tests of the Leggett-Garg type of inequalities that rule out local hidden variable interpretation of the quantum-dot spin dynamics.

We measure time correlators of a spin qubit in an optically active quantumdot beyond the second order. Such higher-order correlators are shown to be directly sensitive to pure quantum effects that cannot be explained within the classical framework. They allow direct determination of ensemble and quantum dephasing times, T_{2}^{*} and T_{2}, using only repeated projective measurements and without the need for coherent spin control. Our method enables studies of purely quantum behavior in solid state systems, including tests of the Leggett-Garg type of inequalities that rule out local hidden variable interpretation of the quantum-dot spin dynamics. PMID:27447523

Charge detection utilizing a highly biased quantum point contact has become the most effective probe for studying few electron quantumdot circuits. Measurements on double and triple quantumdot circuits is performed to clarify a back action role of charge sensing on the confined electrons. The quantum point contact triggers inelastic transitions, which occur quite generally. Under specific device and measurement conditions these transitions manifest themselves as bounded regimes of telegraph noise within a stability diagram. A nonequilibrium transition from artificial atomic to molecular behavior is identified. Consequences for quantum information applications are discussed. PMID:18518321

We report on the observation of periodic conductance oscillations near quantum Hall plateaus in suspended graphene nanoribbons. They are attributed to single quantumdots that are formed in the narrowest part of the ribbon, in the valleys and hills of a disorder potential. In a wide flake with two gates, a double-dot system's signature has been observed. Electrostatic confinement is enabled in single-layer graphene due to the gaps that are formed between the Landau levels, suggesting a way to create gate-defined quantumdots that can be accessed with quantum Hall edge states.We report on the observation of periodic conductance oscillations near quantum Hall plateaus in suspended graphene nanoribbons. They are attributed to single quantumdots that are formed in the narrowest part of the ribbon, in the valleys and hills of a disorder potential. In a wide flake with two gates, a double-dot system's signature has been observed. Electrostatic confinement is enabled in single-layer graphene due to the gaps that are formed between the Landau levels, suggesting a way to create gate-defined quantumdots that can be accessed with quantum Hall edge states. Electronic supplementary information (ESI) available. See DOI: 10.1039/C6NR00187D

Systems and methods are described for fabricating arrays of quantumdots. A method for making a quantumdot device, includes: forming clusters of atoms on a substrate; and charging the clusters of atoms such that the clusters of atoms repel one another. The systems and methods provide advantages because the quantumdots can be ordered with regard to spacing and/or size.

Semiconductor quantumdots are at the forefront of materials science chemistry with applications in biological imaging and photovoltaic technologies. We have developed a simple laboratory experiment to measure the quantum-dot size from fluorescence spectra. A major roadblock of quantum-dot based exercises is the particle synthesis and handling;…

Förster (or fluorescence) resonance energy transfer amongst semiconductor quantumdots (QDs) is reviewed, with particular interest in biosensing applications. The unique optical properties of QDs provide certain advantages and also specific challenges with regards to sensor design, compared to other FRET systems. The brightness and photostability of QDs make them attractive for highly sensitive sensing and long-term, repetitive imaging applications, respectively, but the overlapping donor and acceptor excitation signals that arise when QDs serve as both the donor and acceptor lead to high background signals from direct excitation of the acceptor. The fundamentals of FRET within a nominally homogeneous QD population as well as energy transfer between two distinct colors of QDs are discussed. Examples of successful sensors are highlighted, as is cascading FRET, which can be used for solar harvesting. PMID:26057041

We present a refractometric sensor based on quantumdot-embedded polystyrene microspheres. Optical resonances within a microsphere, known as whispering-gallery modes (WGMs), produce narrow spectral peaks. For sensing applications, spectral shifts of these peaks are sensitive to changes in the local refractive index. In this work, two-photon excited luminescence from the quantumdots couples into several WGMs within the microresonator. By optimizing the detection area, the spectral visibility of the WGMs is improved. The spectral shifts are measured as the surrounding index of the refraction changes. The experimental sensitivity is about five times greater than that predicted by the Mie theory. PMID:19488403

The authors demonstrate theoretically that there exists electromagnetically induced transparency in an asymmetric double quantumdot system using tunneling instead of pump laser. The group velocity slowdown factor is theoretically analyzed as a function of electron tunneling at different broadened linewidths. With feasible parameters for applications to a 100Gbits/s optical network, numerical calculation infers group velocity as low as 300m/s. The scheme is expected to be useful in constructing a variable semiconductor optical buffer based on electromagnetically induced transparency in an asymmetric double quantumdot controlled by voltage.

We develop a numerical model for a defect-containing square lattice of microcavities with embedded ultracold atomic clusters (quantumdots). It is assumed that certain fractions of quantumdots and cavities are absent, which leads to transformation of polariton spectrum of the overall structure. The dispersion relations for polaritonic modes are derived as functions of defect concentrations and on this basis the band gap, the effective masses of lower and upper dispersion branch polaritons as well as their densities of states are evaluated.

We report on the direct experimental determination of the temperature increment caused by laser irradiation in a optical recording media constituted by a polymeric film in which gold nanorods have been incorporated. The incorporation of CdSe quantumdots in the recording media allowed for single beam thermal reading of the on-focus temperature from a simple analysis of the two-photon excited fluorescence of quantumdots. Experimental results have been compared with numerical simulations revealing an excellent agreement and opening a promising avenue for further understanding and optimization of optical writing processes and media.

Luminescent colloidal quantumdots (QDs) possess numerous advantages as fluorophores in biological applications. However, a principal challenge is how to retain the desirable optical properties of quantumdots in aqueous media while maintaining biocompatibility. Because QD photophysical properties are directly related to surface states, it is critical to control the surface chemistry that renders QDs biocompatible while maintaining electronic passivation. For more than a decade, investigators have used diverse strategies for altering the QD surface. This review summarizes the most successful approaches for preparing biocompatible QDs using various chemical ligands. PMID:22247651

The power conversion efficiency of CdSe and CdS quantumdot sensitized solar cells is enhanced by passivation with asymmetrically substituted phthalocyanines. The introduction of the phthalocyanine dye increases the efficiency up to 45% for CdSe and 104% for CdS. The main mechanism causing this improvement is the quantumdot passivation. This study highlights the possibilities of a new generation of dyes designed to be directly linked to QDs instead of the TiO2 electrodes. PMID:25519050

High-efficiency heat engine requires a large output power at the cost of less input heat energy as possible. Here we propose a heat engine composed of serially connected two quantumdots sandwiched between two metallic electrodes. The efficiency of the heat engine can approach the maximum allowable Carnot efficiency ηC. We also find that the strong intradot Coulomb interaction can induce additional work regions for the heat engine, whereas the interdot Coulomb interaction always suppresses the efficiency. Our results presented here indicate a way to fabricate high-efficiency quantum-dot thermoelectric devices.

The exposure of colloidal 2 nm PbS quantumdots to growing continuous wave laser excitation at 532 nm increases the photoluminescence intensity with the square root of the optical stimulus. The results herein in conjunction with previous findings [B. Ullrich and H. Xi, Opt. Lett. 38, 4698 (2013)] advocate the square root trend to be the general limiting function for photo-carrier transport and emission of optically excited nano-sized materials. We further show that the excitation of one electron-hole pair per quantumdot defines the saturation threshold for photoluminescence intensity and dynamic band filling.

We present a refractometric sensor based on quantumdot-embedded polystyrene microspheres. Optical resonances within a microsphere, known as whispering-gallery modes (WGMs), produce narrow spectral peaks. For sensing applications, spectral shifts of these peaks are sensitive to changes in the local refractive index. In this work, two-photon excited luminescence from the quantumdots couples into several WGMs within the microresonator. By optimizing the detection area, the spectral visibility of the WGMs is improved. The spectral shifts are measured as the surrounding index of the refraction changes. The experimental sensitivity is about five times greater than that predicted by the Mie theory. PMID:19488403

We present a model that describes states of photon pairs, which have been generated by biexciton cascade decays of self-assembled quantumdots, the use of which yields a finding that agrees well with the experimental result. Furthermore, we calculate the concurrence and determine the temperature behavior associated with the so-called entanglement sudden death that prevents quantumdots emitting entangled photon pairs at raised temperatures. The relationship between the fine-structure splitting and the sudden death temperature is also provided.

Over the past few years, QuantumDots have been tested in most biotechnological applications that use fluorescence, including DNA array technology, immunofluorescence assays, cell and animal biology. QuantumDots tend to be brighter than conventional dyes, because of the compounded effects of extinction coefficients that are an order of magnitude larger than those of most dyes. Their main advantage resides in their resistance to bleaching over long periods of time (minutes to hours), allowing the acquisition of images that are crisp and well contrasted. This increased photostability is especially useful for three-dimensional (3D) optical sectioning, where a major issue is bleaching of fluorophores during acquisition of successive z-sections, which compromises the correct reconstruction of 3D structures. The long-term stability and brightness of QuantumDots make them ideal candidates also for live animal targeting and imaging. The vast majority of the papers published to date have shown no relevant effects on cells viability at the concentration used for imaging applications; higher concentrations, however, caused some issues on embryonic development. Adverse effects are due to be caused by the release of cadmium, as surface PEGylation of the QuantumDots reduces these issues. A recently published paper shows evidences of an epigenetic effect of QuantumDots treatment, with general histones hypoacetylation, and a translocation to the nucleus of p53. In this study, mice treated with QuantumDots for imaging purposes were analyzed to investigate the impact on protein expression and networking. Differential mono-and bidimensional electrophoresis assays were performed, with the individuation of differentially expressed proteins after intravenous injection and imaging analysis; further, as several authors indicate an increase in reactive oxygen species as a possible mean of damage due to the QuantumDots treatment, we investigated the signalling pathway of APE1/Ref1, a

As the number of applications of quantumdots (QDs) grows, the likelihood of exposure increases. Because these metals have the potential for detrimental environmental and health effects, concerns have been raised over our lack of understanding about the fate of these products. Among various types of QDs, cadmium-based quantumdots attract the greatest attention due to their wide applications. To properly assess the potential risk of cadmium-containing QDs, we summarize the current state of academic knowledge on the toxicity of cadmium-based QDs. PMID:24016111

The spectra of lateral photoconductivity and optical absorption caused by the intraband optical transitions of holes in Ge/Si quantumdots are studied at different lattice temperatures. Polarization-dependent spectral features related to the transitions of holes from the quantumdot (QD) ground state are revealed in the optical spectra. Temperature photoconductivity quenching caused by the reverse trapping of nonequilibrium free holes by the QD bound state is observed. The obtained experimental data make it possible to determine the height of the surface band bending at the QD heterointerface.

We use time-integrated and time-resolved photoluminescence and absorption to characterize the low-temperature optical properties of CdSe quantumdot solids after exchanging native aliphatic ligands for thiocyanate and subsequent thermal annealing. In contrast to trends established at room temperature, our data show that at low temperature the band-edge absorptive bleach is dominated by 1S3/2h hole occupation in the quantumdot interior. We find that our ligand treatments, which bring enhanced interparticle coupling, lead to faster surface state electron trapping, a greater proportion of surface-related photoluminescence, and decreased band-edge photoluminescence lifetimes. PMID:25635923

In this report, a dual-excitation sensing method was developed using aptamer-functionalized quantumdots and upconverting nanoparticles, exhibiting Stokes and anti-Stokes type excitation profiles, respectively. Conjugation of the aptamer-functionalized luminescent nanoparticles with the magnetic beads, comprising short DNA sequences that were partially complementary to the aptamer sequences, enabled facile separation of the analyte-free conjugates for fluorescent measurement. UV-Visible spectroscopy, Circular Dichroism spectroscopy, Dynamic Light Scattering and Polyacrylamide Gel Electrophoresis techniques were used to characterize the aptamer probes developed. The target-specific luminescent conjugates were applied for multiplex detection of model food pathogens, Salmonella typhimurium, and Staphylococcus aureus, in which the fluorescent emission spectra were obtained under UV excitation at 325nm for quantumdots and NIR excitation at 980nm for upconverting nanoparticles, respectively. The dual-excitation strategy was aimed to minimize cross-talk between the luminescent signals for multiplexed detection, and yielded limit of detection values of 16 and 28cfumL(-1) for Staphylococcus aureus, and Salmonella typhimurium, respectively. By employing a greater number of quantumdots and upconverting nanoparticles with non-overlapping fluorescent emissions, the proposed methodology might be exploited further to detect several analytes, simultaneously. PMID:26971274

The similarities between gated quantumdots and the transistors in modern microelectronics--in fabrication methods, physical structure and voltage scales for manipulation--have led to great interest in the development of quantum bits (qubits) in semiconductor quantumdots. Although quantumdot spin qubits have demonstrated long coherence times, their manipulation is often slower than desired for important future applications, such as factoring. Furthermore, scalability and manufacturability are enhanced when qubits are as simple as possible. Previous work has increased the speed of spin qubit rotations by making use of integrated micromagnets, dynamic pumping of nuclear spins or the addition of a third quantumdot. Here we demonstrate a qubit that is a hybrid of spin and charge. It is simple, requiring neither nuclear-state preparation nor micromagnets. Unlike previous double-dot qubits, the hybrid qubit enables fast rotations about two axes of the Bloch sphere. We demonstrate full control on the Bloch sphere with π-rotation times of less than 100 picoseconds in two orthogonal directions, which is more than an order of magnitude faster than any other double-dot qubit. The speed arises from the qubit's charge-like characteristics, and its spin-like features result in resistance to decoherence over a wide range of gate voltages. We achieve full process tomography in our electrically controlled semiconductor quantumdot qubit, extracting high fidelities of 85 per cent for X rotations (transitions between qubit states) and 94 per cent for Z rotations (phase accumulation between qubit states).

Luminescence of amorphous Si quantumdots (Si QDs) in a hydrogenated silicon nitride (SiN{sub x}:H) matrix was examined over a broad range of stoichiometries from Si{sub 3}N{sub 2.08} to Si{sub 3}N{sub 4.14}, to optimize light emission. Plasma-enhanced chemical vapor deposition was used to deposit hydrogenated SiN{sub x} films with excess Si on Si (001) substrates, with stoichiometry controlled by variation of the gas flow rates of SiH{sub 4} and NH{sub 3} gases. The compositional and optical properties were analyzed by Rutherford backscattering spectroscopy, elastic recoil detection, spectroscopic ellipsometry, photoluminescence (PL), time-resolved PL, and energy-filtered transmission electron microscopy. Ultraviolet-laser-excited PL spectra show multiple emission bands from 400 nm (3.1 eV) to 850 nm (1.45 eV) for different Si{sub 3}N{sub x} compositions. There is a red-shift of the measured peaks from ∼2.3 eV to ∼1.45 eV as Si content increases, which provides evidence for quantum confinement. Higher N content samples show additional peaks in their PL spectra at higher energies, which we attribute to defects. We observed three different ranges of composition where Tauc band gaps, PL, and PL lifetimes change systematically. There is an interesting interplay of defect luminescence and, possibly, small Si QD luminescence observed in the intermediate range of compositions (∼Si{sub 3}N{sub 3.15}) in which the maximum of light emission is observed.

Luminescence of amorphous Si quantumdots (Si QDs) in a hydrogenated silicon nitride (SiNx:H) matrix was examined over a broad range of stoichiometries from Si3N2.08 to Si3N4.14, to optimize light emission. Plasma-enhanced chemical vapor deposition was used to deposit hydrogenated SiNx films with excess Si on Si (001) substrates, with stoichiometry controlled by variation of the gas flow rates of SiH4 and NH3 gases. The compositional and optical properties were analyzed by Rutherford backscattering spectroscopy, elastic recoil detection, spectroscopic ellipsometry, photoluminescence (PL), time-resolved PL, and energy-filtered transmission electron microscopy. Ultraviolet-laser-excited PL spectra show multiple emission bands from 400 nm (3.1 eV) to 850 nm (1.45 eV) for different Si3Nx compositions. There is a red-shift of the measured peaks from ˜2.3 eV to ˜1.45 eV as Si content increases, which provides evidence for quantum confinement. Higher N content samples show additional peaks in their PL spectra at higher energies, which we attribute to defects. We observed three different ranges of composition where Tauc band gaps, PL, and PL lifetimes change systematically. There is an interesting interplay of defect luminescence and, possibly, small Si QD luminescence observed in the intermediate range of compositions (˜Si3N3.15) in which the maximum of light emission is observed.

Graphene quantumdots show Coulomb blockade, excited states and their orbital and spin properties have been investigated in high magnetic fields. Most quantumdots fabricated to date are fabricated with electron beam lithography and dry etching which generally leads to uncontrolled and probably rough edges. We demonstrate that devices with reduced bulk disorder fabricated on BN substrates display similar localized states as those fabricated on the more standard SiO2 substrates. For a highly symmetric quantumdot with short tunnel barriers the experimentally detected transport features can be explained by three localized states, 1 in the dot and 2 in the constrictions. A way to overcome edge roughness and the localized states related to this are bilayer devices where a band gap can be induced by suitable top and back gate voltages. By placing bilayer graphene between two BN layers high electronic quality can be achieved as documented by the observation of broken symmetry states in the quantum Hall regime. We discuss how this method can be exploited to achieve smoother and better tunable graphene quantum devices. This work was done in collaboration with D. Bischoff, P. Simonet, A. Varlet, Y. Tian, and T. Ihn.

Colloidal quantumdots, nanocrystal semiconductors that can be cross-linked and assembled into absorbing thin films, are an attractive material for third-generation photovoltaic applications due to low-cost fabrication and bandgap tunability. As a result of their limited charge transport, these solution-processed thin films suffer from a mismatch in absorption length and charge extraction length. Concepts based on the interdigitation of n- and p-doped layers, approaches that reduce the distance photogenerated carriers must travel before extraction, offer promise on overcoming this limitation. In this thesis, I explore and develop techniques to address the absorption-extraction compromise in CQD materials by implementing nano- and micro-structuring techniques to enhance light absorption in the active film. First, I focus on the development of nanomaterials for light guiding/scattering enhancement in CQD films. For this, I develop a nanostructured gold reflector that, when suitably designed, guides light and traps it within the active film. I show that this yields enhanced broadband absorption with more than 4-fold improvement at the most improved wavelength, which translated into a 34% improvement in photocurrent in a working solar cell. I also show that periodic nanostructures employed for absorption enhancement can lead to improvements in solar cell performance. Limitations in device architecture and film formation, however, prevented significant performance advances for these nano-scale approaches. Regardless, these early results pointed me to a new and more impactful strategy. I focus in on realizing micron-scale structured electrodes to enhance absorption, which I show to be considerably more useful in view of the need to extract charge carriers with high efficiency. I discover that conformal film formation atop these structured electrodes is an absolute prerequisite to enhancing performance. These devices, which I term micro-pyramid CQD cells, provide a 24

We demonstrate a quantumdotquantum cascade photodetector with a hybrid active region of InAs quantumdots and an InGaAs quantum well, which exhibited a temperature independent response at 4.5 μm. The normal incident responsivity reached 10.3 mA/W at 120 K and maintained a value of 9 mA/W up to 260 K. It exhibited a specific detectivity above 1011 cm Hz1/2 W-1 at 77 K, which remained at 108 cm Hz1/2 W-1 at 260 K. We ascribe the device's good thermal stability of infrared response to the three-dimensional quantum confinement of the InAs quantumdots incorporated in the active region.

The Josephson effect describes supercurrent flowing through a junction connecting two superconducting leads by a thin barrier. This current is driven by a superconducting phase difference ϕ between the leads. In the presence of chiral and time-reversal symmetry of the Cooper pair tunnelling process, the current is strictly zero when ϕ vanishes. Only if these underlying symmetries are broken can the supercurrent for ϕ = 0 be finite. This corresponds to a ground state of the junction being offset by a phase ϕ0, different from 0 or π. Here, we report such a Josephson ϕ0-junction based on a nanowire quantumdot. We use a quantum interferometer device to investigate phase offsets and demonstrate that ϕ0 can be controlled by electrostatic gating. Our results may have far-reaching implications for superconducting flux- and phase-defined quantum bits as well as for exploring topological superconductivity in quantumdot systems.

The possibility to make 10 nm scale, and low-disorder, suspended graphene devices would open up many possibilities to study and make use of strongly coupled quantum electronics, quantum mechanics, and optics. We present a versatile method, based on the electromigration of gold-on-graphene bow-tie bridges, to fabricate low-disorder suspended graphene junctions and quantumdots with lengths ranging from 6 nm up to 55 nm. We control the length of the junctions, and shape of their gold contacts by adjusting the power at which the electromigration process is allowed to avalanche. Using carefully engineered gold contacts and a nonuniform downward electrostatic force, we can controllably tear the width of suspended graphene channels from over 100 nm down to 27 nm. We demonstrate that this lateral confinement creates high-quality suspended quantumdots. This fabrication method could be extended to other two-dimensional materials. PMID:25490053

Using background-free detection of spin-state-dependent resonance fluorescence from a single-electron charged quantumdot with an efficiency of 0.1%, we realize a classical single spin-photon interface where the detection of a scattered photon with 300 ps time resolution projects the quantumdot spin to a definite spin eigenstate with fidelity exceeding 99%. The bunching of resonantly scattered photons reveals information about electron spin dynamics. High-fidelity fast spin-state initialization heralded by a single photon enables the realization of quantum information processing tasks such as nondeterministic distant spin entanglement. Given that we could suppress the measurement backaction to well below the natural spin-flip rate, realization of a quantum nondemolition measurement of a single spin could be achieved by increasing the fluorescence collection efficiency by a factor exceeding 10 using a photonic nanostructure. PMID:20867763

The circular-sector quantum-billiard problem is studied. Numerical evaluation of the zeros of first-order Bessel functions finds that there is an abrupt change in the nodal-line structure of the first excited state of the system (equivalently, second eigenstate of the Laplacian) at the critical sector-angle θc=0.354π. For sector-angle θ0, in the domain 0circular-arc segment. For θcquantum-billiard configurations.

Successful implementation of technology using self-forming semiconductor QuantumDots (QDs) has already demonstrated that temperature independent Dirac-delta density of states can be exploited in low current threshold QD lasers and QD infrared photodetectors.

Conductance through quantumdots at low temperature exhibits random but repeatable fluctuations arising from quantum interference of electrons. The observed fluctuations follow universal statistics arising from the underlying universality of quantum chaos. Random matrix theory (RMT) has provided an accurate description of the observed universal conductance fluctuations (UCF) in "open" quantumdots (device conductance ≥e 2/h). The focus of this thesis is to search for and decipher the underlying origin of similar universal properties in "closed" quantumdots (device conductance ≤e2/ h). A series of experiments is presented on electronic ground state properties measured via conductance measurements in Coulomb blockaded quantumdots. The statistics of Coulomb blockade (CB) peak heights with zero and non-zero magnetic field measured in various devices agree qualitatively with predictions from Random Matrix Theory (RMT). The standard deviation of the peak height fluctuations for non-zero magnetic field is lower than predicted by RMT; the temperature dependence of the standard deviation of the peak height for non-zero magnetic field is also measured. The second experiment summarizes the statistics of CB peak spacings. The peak spacing distribution width is observed to be on the order of the single particle level spacing, Delta, for both zero and non-zero magnetic field. The ratio of the zero field peak spacing distribution width to the non-zero field peak spacing distribution width is ˜1.2; this is good agreement with predictions from spin-resolved RMT predictions. The standard deviation of the non-zero magnetic field peak spacing distribution width shows a T-1/2 dependence in agreement with a thermal averaging model. The final experiment summarizes the measurement of the peak height correlation length versus temperature for various quantumdots. The peak height correlation length versus temperature saturates in small quantumdots, suggesting spectral scrambling

We demonstrate the optical levitation or trapping in helium gas of a single quantumdot (QD) within a liquid droplet. Bright single photon emission from the levitated QD in the droplet was observed for more than 200 s. The observed photon count rates are consistent with the value theoretically estimated from the two-photon-action cross section. This Letter presents the realization of an optically levitated solid-state quantum emitter. PMID:25768143

We studied the quantum correlation of the photon pairs generated by biexciton cascade decays of self-assembled quantumdots, and determined the correlation sudden-change temperature, which is shown to be independent of the background noise, far lower than the entanglement sudden-death temperature, and therefore, easier to be observed in experiments. The relationship between the fine-structure splitting and the sudden-change temperature is also provided.

Quantumdot solar cells, as a promising candidate for the next generation solar cell technology, have received tremendous attention in the last 10 years. Some recent developments in epitaxy growth and device structures have opened up new avenues for practical quantumdot solar cells. Unfortunately, the performance of quantumdot solar cells is often plagued by marginal photon absorption. In this review, we focus on the recent progress made in enhancing optical absorption in quantumdot solar cells, including optimization of quantumdot growth, improving the solar cells structure, and engineering light trapping techniques. PMID:27216604

Quantumdot solar cells, as a promising candidate for the next generation solar cell technology, have received tremendous attention in the last 10 years. Some recent developments in epitaxy growth and device structures have opened up new avenues for practical quantumdot solar cells. Unfortunately, the performance of quantumdot solar cells is often plagued by marginal photon absorption. In this review, we focus on the recent progress made in enhancing optical absorption in quantumdot solar cells, including optimization of quantumdot growth, improving the solar cells structure, and engineering light trapping techniques.

Semiconductor quantumdots (QDs) are attractive candidates for scalable solid state implementations of quantum information processing based on electron spin states, where a crucial requirement for practical devices is to have efficient and tunable spin coupling between them. We focus on recent femtosecond time-resolved Faraday rotation studies of self-assembled multilayer spintronic devices based on colloidal quantumdots bridged by conjugated molecules (M. Ouyang et al., Science 301, 1074 (2003)). The data reveal the instantaneous transfer of spin coherence through conjugated molecular bridges spanning quantumdots of different size over a broad range of temperature. The room temperature spin transfer efficiency exceeds 20%, which approximately doubles the value measured at T=4.5K. A molecular π-orbital mediated spin coherence transfer mechanism is proposed to provide a qualitative insight into the experimental observations, suggesting a correlation between the stereochemistry of molecules and the transfer process. The results show that conjugated molecules can be used not only as physical links for the assembly of functional networks, but also as efficient channels for shuttling quantum information. This class of structures may be useful as two-spin quantum devices operating at ambient temperatures and may offer promising opportunities for future versatile molecule-based spintronic technologies.

We study decoherence of an electron spin qubit that is being transported in a moving quantumdot. Our focus is on spin relaxation due to phonon noise through the spin-orbit interaction. We find that the effective magnetic field caused by the motion of the electron can either enhance or suppress spin relaxation depending on the angle between the moving direction and the external magnetic field. At low external magnetic field (BAlt 0 . 5 T), the suppression effect can be significant, which indicates that a moving quantumdot can maintain spin coherence better than a static dot. We also find that the spin relaxation rate is not a monotonically increasing function of the applied magnetic field when the motion of the electron is taken into account. We thank financial support by US ARO and NSF PIF.

The excitation spectrum of a quantumdot varies with the addition of electrons, as successive single-particle eigenstates become filled in the ground state and so cannot accomodate additional electrons. Previous experiments have observed that each spatial state becomes unavailable for transport of further electrons after only one electron has occupied it. We have investigated state occupancy in the excitation spectrum of a small (200 nm X 200 nm) quantumdot laterally defined by capacitively coupled gate electrodes in a GaAs/AlGaAs heterostructure. For our dots, quantized level spacing Δ E ≈ 300 μeV and charging energy Ec ≈ 2 meV. We have studied the evolution of features in the excitation spectrum with magnetic field and equilibrium occupancy and have identified the pattern of spins for the added electrons. These results test the applicability of the spin-degenerate constant interaction picture as well as its limitations.

The mechanisms of the nonradiative recombination of excitons in neutral and charged quantumdots based on II–VI semimagnetic semiconductors are investigated. It is shown that, along with the dipole–dipole and direct-exchange mechanisms, there is one more mechanism referred to as the indirect-exchange mechanism and related to sp–d mixing. The selection rules for nonradiative recombination by exchange mechanisms are subsequently derived. The dependence of the efficiency of all recombination mechanisms on the quantum-dot size is studied. The experimentally observed growth in the intracenter photoluminescence intensity with decreasing size of dots and nanocrystals is accounted for. Methods for experimental determination of the contributions of different mechanisms to nonradiative recombination are discussed.

By using a simple model we consider single-channel transmission through a double quantumdot that consists of two single dots coupled by a wire of finite length L . Each of the two single dots is characterized by a few energy levels only, and the wire is assumed to have only one level whose energy depends on the length L . The transmission is described by using S matrix theory and the effective non-Hermitian Hamilton operator H(eff) of the system. The decay widths of the eigenstates of H(eff) depend strongly on energy. The model explains the origin of the transmission zeros of the double dot that is considered by us. Mostly, they are caused by (destructive) interferences between neighboring levels and are of first order. When, however, both single dots are identical and their transmission zeros are of first order, those of the double dot are of second order. First-order transmission zeros cause phase jumps of the transmission amplitude by pi, while there are no phase jumps related to second-order transmission zeros. In this latter case, a phase jump occurs due to the fact that the width of one of the states vanishes when crossing the energy of the transmission zero. The parameter dependence of the widths of the resonance states is determined by the spectral properties of the two single dots. PMID:15903767

We propose an implementation of quantum neural networks using an array of quantumdots with dipole-dipole interactions. We demonstrate that this implementation is both feasible and versatile by studying it within the framework of GaAs based quantumdot qubits coupled to a reservoir of acoustic phonons. Using numerically exact Feynman integral calculations, we have found that the quantum coherence in our neural networks survive for over a hundred ps even at liquid nitrogen temperatures (77 K), which is three orders of magnitude higher than current implementations, which are based on SQUID-based systems operating at temperatures in the mK range.

Schemes for giant fifth-order nonlinearity via tunneling in both linear and triangular triple quantumdots are proposed. In both configurations, the real part of the fifth-order nonlinearity can be greatly enhanced, and simultaneously the absorption is suppressed. The analytical expression and the dressed states of the system show that the two tunnelings between the neighboring quantumdots can induce quantum interference, resulting in the giant higher-order nonlinearity. The scheme proposed here may have important applications in quantum information processing at low light level.

Schemes for giant fifth-order nonlinearity via tunneling in both linear and triangular triple quantumdots are proposed. In both configurations, the real part of the fifth-order nonlinearity can be greatly enhanced, and simultaneously the absorption is suppressed. The analytical expression and the dressed states of the system show that the two tunnelings between the neighboring quantumdots can induce quantum interference, resulting in the giant higher-order nonlinearity. The scheme proposed here may have important applications in quantum information processing at low light level.

The photoluminescence (PL) origin of bright blue emission arising from intrinsic states in graphene quantumdots (GQDs) is investigated. The bright PL of intercalatively acquired GQDs is attributed to favorably formed subdomains composed of four to seven carbon hexagons. Random and harsh oxidation which hinders the energetically favorable formation of subdomains causes weak and redshifted PL. PMID:27153519

Glaucoma is a leading cause of blindness in the world, often associated with elevated eye pressure. Currently, all glaucoma treatments aim to lower eye pressure by improving fluid exit from the eye. We recently reported the presence of lymphatics in the human eye. The lymphatic circulation is known to drain fluid from organ tissues and, as such, lymphatics may also play a role in draining fluid from the eye. We investigated whether lymphatic drainage from the eye is present in mice by visualizing the trajectory of quantumdots once injected into the eye. Whole-body hyperspectral fluorescence imaging was performed in 17 live mice. In vivo imaging was conducted prior to injection, and 5, 20, 40 and 70 min, and 2, 6 and 24 h after injection. A quantumdot signal was observed in the left neck region at 6 h after tracer injection into the eye. Examination of immunofluorescence-labelled sections using confocal microscopy showed the presence of a quantumdot signal in the left submandibular lymph node. This is the first direct evidence of lymphatic drainage from the mouse eye. The use of quantumdots to image this lymphatic pathway in vivo is a novel tool to stimulate new treatments to reduce eye pressure and prevent blindness from glaucoma.

The performance of thermoelectric energy harvesters can be improved by nanostructures that exploit inelastic transport processes. One prototype is the three-terminal hopping thermoelectric device where electron hopping between quantum-dots are driven by hot phonons. Such three-terminal hopping thermoelectric devices have potential in achieving high efficiency or power via inelastic transport and without relying on heavy-elements or toxic compounds. We show in this work how output power of the device can be optimized via tuning the number and energy configuration of the quantum-dots embedded in parallel nanowires. We find that the staircase energy configuration with constant energy-step can improve the power factor over a serial connection of a single pair of quantum-dots. Moreover, for a fixed energy-step, there is an optimal length for the nanowire. Similarly for a fixed number of quantum-dots there is an optimal energy-step for the output power. Our results are important for future developments of high-performance nanostructured thermoelectric devices. PMID:27550093

In the past few decades, technology has made immeasurable strides to enable visualization, identification, and quantitation in biological systems. Many of these technological advancements are occurring on the nanometer scale, where multiple scientific disciplines are combining to create new materials with enhanced properties. The integration of inorganic synthetic methods with a size reduction to the nano-scale has lead to the creation of a new class of optical reporters, called quantumdots. These semiconductor quantumdot nanocrystals have emerged as an alternative to organic dyes and fluorescent proteins, and are brighter and more stable against photobleaching than standard fluorescent indicators. Quantumdots have tunable optical properties that have proved useful in a wide range of applications from multiplexed analysis such as DNA detection and cell sorting and tracking, to most recently demonstrating promise for in vivo imaging and diagnostics. This review provides an in-depth discussion of past, present, and future trends in quantumdot use with an emphasis on in vivo imaging and its related applications. PMID:19333416

Colloidal quantumdots have grown in interest as materials for light amplification and lasing in view of their bright photoluminescence, convenient solution processing and size-controlled spectral tunability. To date, lasing in colloidal quantumdot solids has been limited to the nanosecond temporal regime, curtailing their application in systems that require more sustained emission. Here we find that the chief cause of nanosecond-only operation has been thermal runaway: the combination of rapid heat injection from the pump source, poor heat removal and a highly temperature-dependent threshold. We show microsecond-sustained lasing, achieved by placing ultra-compact colloidal quantumdot films on a thermally conductive substrate, the combination of which minimizes heat accumulation. Specifically, we employ inorganic-halide-capped quantumdots that exhibit high modal gain (1,200 cm-1) and an ultralow amplified spontaneous emission threshold (average peak power of ~50 kW cm-2) and rely on an optical structure that dissipates heat while offering minimal modal loss.

Colloidal quantumdots have grown in interest as materials for light amplification and lasing in view of their bright photoluminescence, convenient solution processing and size-controlled spectral tunability. To date, lasing in colloidal quantumdot solids has been limited to the nanosecond temporal regime, curtailing their application in systems that require more sustained emission. Here we find that the chief cause of nanosecond-only operation has been thermal runaway: the combination of rapid heat injection from the pump source, poor heat removal and a highly temperature-dependent threshold. We show microsecond-sustained lasing, achieved by placing ultra-compact colloidal quantumdot films on a thermally conductive substrate, the combination of which minimizes heat accumulation. Specifically, we employ inorganic-halide-capped quantumdots that exhibit high modal gain (1,200 cm−1) and an ultralow amplified spontaneous emission threshold (average peak power of ∼50 kW cm−2) and rely on an optical structure that dissipates heat while offering minimal modal loss. PMID:26493282

The performance of thermoelectric energy harvesters can be improved by nanostructures that exploit inelastic transport processes. One prototype is the three-terminal hopping thermoelectric device where electron hopping between quantum-dots are driven by hot phonons. Such three-terminal hopping thermoelectric devices have potential in achieving high efficiency or power via inelastic transport and without relying on heavy-elements or toxic compounds. We show in this work how output power of the device can be optimized via tuning the number and energy configuration of the quantum-dots embedded in parallel nanowires. We find that the staircase energy configuration with constant energy-step can improve the power factor over a serial connection of a single pair of quantum-dots. Moreover, for a fixed energy-step, there is an optimal length for the nanowire. Similarly for a fixed number of quantum-dots there is an optimal energy-step for the output power. Our results are important for future developments of high-performance nanostructured thermoelectric devices. PMID:27550093

We demonstrate the first quantumdot (QD) laser on a silicon substrate with efficient coupling of light to a silicon waveguide under the QD gain region. Continuous wave operation up to 100 °C and multiwavelength operation are demonstrated, paving the way towards highly efficient CMOS-compatible, uncooled, WDM sources. PMID:27410883

Bound and resonance states of helium atom have been investigated inside a quantumdot by using explicitly correlated Hylleraas type basis set within the framework of stabilization method. To be specific, precise energy eigenvalues of bound 1sns (1Se) (n = 1-6) states and the resonance parameters i.e. positions and widths of 1Se states due to 2sns (n = 2-5) and 2pnp (n = 2-5) configurations of confined helium below N = 2 ionization threshold of He+ have been estimated. The two-parameter (Depth and Width) finite oscillator potential is used to represent the confining potential due to the quantumdot. It has been explicitly demonstrated that the electronic structural properties become sensitive functions of the dot size. It is observed from the calculations of ionization potential that the stability of an impurity ion within a quantumdot may be manipulated by varying the confinement parameters. A possibility of controlling the autoionization lifetime of doubly excited states of two-electron ions by tuning the width of the quantum cavity is also discussed here. TKM Gratefully Acknowledges Financial Support under Grant No. 37(3)/14/27/2014-BRNS from the Department of Atomic Energy, BRNS, Government of India. SB Acknowledges Financial Support under Grant No. PSW-160/14-15(ERO) from University Grants Commission, Government of India

ZnO nanorods grown by hydrothermal technique are of great interest for potential applications in photovoltaic and optoelectronic devices. In this study we investigate the optimization of the optical absorption properties by a low temperature, chemical bath deposition technique. Our group fabricated nanorods on indium tin oxide (ITO) substrate with precursor solution of zinc nitrate hexahydrate and hexamethylenetramine (1:1 molar ratio) at 95C for 9 hours. In order to optimize the light absorption characteristics of ZnO nanorods, CdSe/ZnS core-shell quantumdots (QDs) of various diameters were attached to the surface of ZnO nanostructures grown on ITO and gold-coated silicon substrates. Density of quantumdots was varied by controlling the number drops on the surface of the ZnO nanorods. For a 0.1 M concentration of QDs of 10 nm diameter, the PL intensity at 385 nm increased as the density of the quantumdots on ZnO nanostructures was increased. For quantumdots at 1 M concentration, the PL intensity at 385 nm increased at the beginning and then decreased at higher density. We will discuss the observed changes in PL intensity with QD concentration with ZnO-QD band structure and recombination-diffusion processes taking place at the interface.

The exposure of living systems to nanoparticles is inevitable due to a dramatic increase in their release into the environment, the most likely pathways being through inhalation, ingestion and skin uptake. The extremely small size of the nanoparticles may facilitate their tissue and cellular uptake by plants and animals, resulting in either positive (drug delivery, antioxidation) or negative (toxicity, cellular dysfunction) effects. Here we report the effects of quantumdots uptake by algae, the single-celled plant species and major food sources for aquatic organisms. In our studies, the presence of quantumdots in algal cells was detected using fluorescence microscopy and electron microscopy. Using spectrophotometry we found a supralinear increase of the uptake with the concentration of quantumdots, with a saturation of the uptake occurring beyond a concentration of 15 mg/mL. Using a bicarbonate indicator we further evaluated the effects of quantumdots uptake on algal photosynthesis and respiration. Such study facilitates our understanding of the environmental impact of nanomaterials.

Colloidal semiconductor quantumdots (QDs) have evolved beyond scientific novelties and are transitioning into bona fide analytical tools. We describe the burgeoning role of QDs in many different fields of bioanalyses and highlight the advantages afforded by their unique physical and optical properties. PMID:21928771

A process has been devised for making ternary (specifically, CuInS2) nanocrystals for use as quantumdots (QDs) in a contemplated next generation of high-efficiency solar photovoltaic cells. The process parameters can be chosen to tailor the sizes (and, thus, the absorption and emission spectra) of the QDs.

Zinc oxide quantumdots (ZnO QDs) are nanoparticles of purified powdered ZnO. The ZnO QDs were directly added into liquid foods or coated on the surface of glass jars using polylactic acid (PLA) as a carrier. The antimicrobial activities of ZnO QDs against Listeria monocytogenes, Salmonella Enteriti...

Summary Recently, a new simple and fast method for the synthesis of InP quantumdots by using phosphine as phosphorous precursor and myristic acid as surface stabilizer was reported. Purification after synthesis is necessary to obtain samples with good optical properties. Two methods of purification were compared and the surface processes which occur during purification were studied. Traditional precipitation with acetone is accompanied by a small increase in photoluminescence. It occurs that during the purification the hydrolysis of the indium precursor takes place, which leads to a better surface passivation. The electrophoretic purification technique does not increase luminescence efficiency but yields very pure quantumdots in only a few minutes. Additionally, the formation of In(OH)3 during the low temperature synthesis was explained. Purification of quantumdots is a very significant part of postsynthetical treatment that determines the properties of the material. But this subject is not sufficiently discussed in the literature. The paper is devoted to the processes that occur at the surface of quantumdots during purification. A new method of purification, electrophoresis, is investigated and described in particular. PMID:25161857

Several logical qubits and quantum gates have been proposed for semiconductor quantumdots controlled by voltages applied to top gates. The different schemes can be difficult to compare meaningfully. Here we develop a theoretical framework to evaluate disparate qubit-gating schemes on an equal footing. We apply the procedure to two types of double-dot qubits: the singlet–triplet and the semiconducting quantumdot hybrid qubit. We investigate three quantum gates that flip the qubit state: a DC pulsed gate, an AC gate based on logical qubit resonance, and a gate-like process known as stimulated Raman adiabatic passage. These gates are all mediated by an exchange interaction that is controlled experimentally using the interdot tunnel coupling g and the detuning ϵ, which sets the energy difference between the dots. Our procedure has two steps. First, we optimize the gate fidelity (f) for fixed g as a function of the other control parameters; this yields an that is universal for different types of gates. Next, we identify physical constraints on the control parameters; this yields an upper bound that is specific to the qubit-gate combination. We show that similar gate fidelities should be attainable for singlet-triplet qubits in isotopically purified Si, and for hybrid qubits in natural Si. Considerably lower fidelities are obtained for GaAs devices, due to the fluctuating magnetic fields ΔB produced by nuclear spins. PMID:24255105

Two-dimensional (2D) dynamic photonic crystal regime has been utilized to investigate self-diffraction effect and nonlinear optical properties of excitons in CdSe/ZnS colloidal quantumdots (QDs). Self-diffraction at 2D photonic crystal arises for three intersecting beams of Nd+3:YAG laser second harmonic in the case of one-photon resonant excitation of the exciton (electron - hole) transition QDs. The relaxation time of excited excitons has been measured by pump and probe technique at induced one-dimensional transient diffraction grating. Two-exponential decay with initial fast and slow parts was discovered. Self-action effect has been discovered in the case of stationary resonant excitation of excitons in CdSe/ZnS QDs by the beam of second harmonic of powerful 12-nanosecond laser pulses. The bleaching of exciton absorption and the creation of transparency channel (this effect provokes self-diffraction of the second harmonic beam) was explained by the dominating coexisting and competing processes of state filling in stationary excited quantumdots and Stark-shift of exciton spectral band. The peculiarities of the influence of these processes at the change of exciton absorption in quantumdots in the case of different detuning from exciton resonance (quantumdots with different size have been used) was analyzed.

To make photonic quantum information a reality, a number of extraordinary challenges need to be overcome. One challenge is to achieve large arrays of reproducible `entangled' photon generators, while maintaining compatibility for integration with optical devices and detectors. Semiconductor quantumdots are potentially ideal for this as they allow photons to be generated on demand without relying on probabilistic processes. Nevertheless, most quantum-dot systems are limited by their intrinsic lack of symmetry, which allows only a small number (typically 1 out of 100, or worse) of good dots to be achieved per chip. The recent retraction of Mohan et al. seemed to question the very possibility of simultaneously achieving site control and high symmetry. Here, we show that with a new family of (111)-grown pyramidal site-controlled InGaAs1-δNδ quantumdots it is possible to overcome previous hurdles and obtain areas with up to 15% of polarization-entangled photon emitters, with fidelities as high as 0.721 +/- 0.043.

A proposed integrated circuit based on quantum-dot cellular automata (QCA) would function as a bit-serial adder. This circuit would serve as a prototype building block for demonstrating the feasibility of quantum-dots computing and for the further development of increasingly complex and increasingly capable quantum-dots computing circuits. QCA-based bit-serial adders would be especially useful in that they would enable the development of highly parallel and systolic processors for implementing fast Fourier, cosine, Hartley, and wavelet transforms. The proposed circuit would complement the QCA-based circuits described in "Implementing Permutation Matrices by Use of QuantumDots" (NPO-20801), NASA Tech Briefs, Vol. 25, No. 10 (October 2001), page 42 and "Compact Interconnection Networks Based on QuantumDots" (NPO-20855), which appears elsewhere in this issue. Those articles described the limitations of very-large-scale-integrated (VLSI) circuitry and the major potential advantage afforded by QCA. To recapitulate: In a VLSI circuit, signal paths that are required not to interact with each other must not cross in the same plane. In contrast, for reasons too complex to describe in the limited space available for this article, suitably designed and operated QCA-based signal paths that are required not to interact with each other can nevertheless be allowed to cross each other in the same plane without adverse effect. In principle, this characteristic could be exploited to design compact, coplanar, simple (relative to VLSI) QCA-based networks to implement complex, advanced interconnection schemes. To enable a meaningful description of the proposed bit-serial adder, it is necessary to further recapitulate the description of a quantum-dot cellular automation from the first-mentioned prior article: A quantum-dot cellular automaton contains four quantumdots positioned at the corners of a square cell. The cell contains two extra mobile electrons that can tunnel (in the

We report the fabrication of single and double hole quantumdots using a double-layer-gate design on an undoped accumulation mode [Formula: see text]/GaAs heterostructure. Electrical transport measurements of a single quantumdot show varying addition energies and clear excited states. In addition, the two-level-gate architecture can also be configured into a double quantumdot with tunable inter-dot coupling. PMID:27389108

We report the fabrication of single and double hole quantumdots using a double-layer-gate design on an undoped accumulation mode {{Al}}x{{Ga}}1-x{As}/GaAs heterostructure. Electrical transport measurements of a single quantumdot show varying addition energies and clear excited states. In addition, the two-level-gate architecture can also be configured into a double quantumdot with tunable inter-dot coupling.

A top-gated single-wall carbon nanotube is used to define three coupled quantumdots in series between two electrodes. The additional electron number on each quantumdot is controlled by top-gate voltages allowing for current measurements of single, double, and triple quantumdot stability diagrams. Simulations using a capacitor model including tunnel coupling between neighboring dots captures the observed behavior with good agreement. Furthermore, anticrossings between indirectly coupled levels and higher order cotunneling are discussed. PMID:18314966

The time evolution of the quantum entropy in a coherently driven triple quantumdot molecule is investigated. The entanglement of the quantumdot molecule and its spontaneous emission field is coherently controlled by the gate voltage and the rate of an incoherent pump field. The degree of entanglement between a triple quantumdot molecule and its spontaneous emission fields is decreased by increasing the tunneling parameter.

To reduce the charge noise of a modulation-doped GaAs/AlGaAs quantumdot, we have fabricated shallow-etched GaAs/AlGaAs quantumdots using the wet-etching method to study the effects of two-dimensional electron gas (2DEG) underneath the metallic gates. The low-frequency 1/f noise in the Coulomb blockade region of the shallow-etched quantumdot is compared with a non-etched quantumdot on the same wafer. The average values of the gate noise are approximately 0.5 μeV in the shallow-etched quantumdot and 3 μeV in the regular quantumdot. Our results show the quantumdot low-frequency charge noise can be suppressed by the removal of the 2DEG underneath the metallic gates, which provides an architecture for noise reduction.

Strain engineering during the capping of III-V quantumdots has been explored as a means to control the height of strained self-assembled quantumdots. Results of Kinetic Monte Carlo simulations are confronted with cross-sectional Scanning Tunnel Microscopy (STM) measurements performed on InAs quantumdots grown by molecular beam epitaxy. We studied InAs quantumdots that are capped by In{sub x}Ga{sub (1−x)}As layers of different indium compositions. Both from our realistic 3D kinetic Monte Carlo simulations and the X-STM measurements on real samples, a trend in the height of the capped quantumdot is found as a function of the lattice mismatch between the quantumdot material and the capping layer. Results obtained on additional material combinations show a generic role of the elastic energy in the control of the quantumdot morphology by strain engineering during capping.

`In this chapter, we introduced and analyze techniques that allow truly secure secret key sharing over long distances, using public, open channels, where the laws of quantum mechanics ensure the security of the long distance key sharing - an idea generally referred to as the essence of a quantum repeater. We describe several proof-of-principle experiments where technology based on self-assembled quantumdots is used as the backbone of a future quantum repeater.'

The present work is concerned with a report on the effects of Pauli, Rashba and Dresselhaus spin-orbit interactions (SOI) on the energy levels of a 2D circular hydrogenic quantum anti-dot(QAD). To pursue this aim, we first present a brief review on the analytical solutions to the Schrödinger equation of electronic states in a quantum anti-dot when a hydrogenic donor is placed at the center, revealing the degeneracies involved in the ground, first and second excited states. We then proceed by adding the aforementioned spin-orbit interactions to the Hamiltonian and treat them as perturbation, thereby, calculating the energy shifts to the first three states. As we show, the Rashba spin-orbit interaction gives rise to a shift in the energies of the ground and second excited states, while it partially lifts the degeneracy of the first excited state. Our calculations also indicate that the Dresselhaus effect, while keeping the degeneracy of the ground and second excited states intact, removes the degeneracy of the first excited state in the opposite sense. The Pauli spin-orbit interaction, on the other hand, is diagonal in the appropriate bases, and thus its effect is readily calculated. The results show that degeneracy of ℓ = 0 (prevailing in the ground and second excited state) remains but the degeneracy of ℓ = 1 (prevailing in the first excited state) is again partially lifted. Moreover, we present the energy corrections due to the three spin-orbit interactions as functions of anti-dot's radius, Rashba and Dresselhaus strengths discussing how they affect the corresponding states. The material presented in the article conceives the possibility of generating spin currents in the hydrogenic circular anti-dots.

Quantumdots embedded within nanowires represent one of the most promising technologies for applications in quantum photonics. Whereas the top-down fabrication of such structures remains a technological challenge, their bottom-up fabrication through self-assembly is a potentially more powerful strategy. However, present approaches often yield quantumdots with large optical linewidths, making reproducibility of their physical properties difficult. We present a versatile quantum-dot-in-nanowire system that reproducibly self-assembles in core-shell GaAs/AlGaAs nanowires. The quantumdots form at the apex of a GaAs/AlGaAs interface, are highly stable, and can be positioned with nanometre precision relative to the nanowire centre. Unusually, their emission is blue-shifted relative to the lowest energy continuum states of the GaAs core. Large-scale electronic structure calculations show that the origin of the optical transitions lies in quantum confinement due to Al-rich barriers. By emitting in the red and self-assembling on silicon substrates, these quantumdots could therefore become building blocks for solid-state lighting devices and third-generation solar cells. PMID:23377293

H. Tetsuka and co-workers develop a versatile technique to tune the energy levels and energy gaps of nitrogen-functionalized graphene quantumdots (NGQDs) continuously through molecular structure design, as described on page 4632. The incorporation of layers of NGQDs into the structures markedly improves the performance of optoelectronic devices. PMID:27281048

Chaos presents a striking and fascinating phenomenon of nonlinear systems. A common aspect of such systems is the presence of feedback that couples the output signal partially back to the input. Feedback coupling can be well controlled in optoelectronic devices such as conventional semiconductor lasers that provide bench-top platforms for the study of chaotic behaviour and high bit rate random number generation. Here we experimentally demonstrate that chaos can be observed for quantum-dot microlasers operating close to the quantum limit at nW output powers. Applying self-feedback to a quantum-dot microlaser results in a dramatic change in the photon statistics wherein strong, super-thermal photon bunching is indicative of random-intensity fluctuations associated with the spiked emission of light. Our experiments reveal that gain competition of few quantumdots in the active layer enhances the influence of self-feedback and will open up new avenues for the study of chaos in quantum systems. PMID:21694714

In the present work, we investigate the transport through a T-shaped double quantumdot system coupled to two normal leads and to a superconducting lead. We study the role of the superconducting lead in the quantum interferometric features of the double quantumdot and by means of a slave boson mean field approximation at low temperature regime. We inquire into the influence of intradot interactions in the electronic properties of the system as well. Our results show that Fano resonances due to Andreev bound states are exhibited in the transmission from normal to normal lead as a consequence of quantum interference and proximity effect. This Fano effect produced by Andreev bound states in a side quantumdot was called Fano-Andreev effect, which remains valid even if the electron-electron interaction are taken into account, that is, the Fano-Andreev effect is robust against e-e interactions even in Kondo regime. We acknowledge the financial support from FONDECYT program Grants No. 3140053 and 11400571.

We propose an experimental scheme to implement a strong photon blockade with a single quantumdot coupled to a nanocavity. The photon blockade effect can be tremendously enhanced by driving the cavity and the quantumdot simultaneously with two classical laser fields. This enhancement of photon blockade is ascribed to the quantum interference effect to avoid two-photon excitation of the cavity field. Comparing with Jaynes-Cummings model, the second-order correlation function at zero time delay g(2)(0) in our scheme can be reduced by two orders of magnitude and the system sustains a large intracavity photon number. A red (blue) cavity-light detuning asymmetry for photon quantum statistics with bunching or antibunching characteristics is also observed. The photon blockade effect has a controllable flexibility by tuning the relative phase between the two pumping laser fields and the Rabi coupling strength between the quantumdot and the pumping field. Moreover, the photon blockade scheme based on quantum interference mechanism does not require a strong coupling strength between the cavity and the quantumdot, even with the pure dephasing of the system. This simple proposal provides an effective way for potential applications in solid state quantum computation and quantum information processing. PMID:25783560

Microwave resonators coupled to quantum systems have been used for fast dispersive measurement in several different architectures in solid state and atomic physics. The electronic states of a semiconductor quantumdot represent a promising candidate for quantum information processing. Our work is geared toward developing a fast, non-demolition readout of a semiconductor qubit in silicon through coupling to a superconducting microwave resonator. We report progress on a novel design of a lateral This work is supported by the DARPA QuEST program.

Lead Chalcogenides (PbS, PbSe, PbTe) quantumdots (QDs) are ideal for fundamental studies of strongly quantum confined systems with possible technological applications. Tunable electronic transitions at near--infrared wavelengths can be obtained with these QDs. Applications of lead chalcogenides encompass quite a good number of important field viz. the fields of telecommunications, medical electronics, optoelectronics etc. Very recently, it has been proposed that 'memristor'(Memory resistor) can be realized in nanoscale systems with coupled ionic and electronic transports. The hystersis characteristics of 'memristor' are observed in many nanoscale electronic devices including semiconductor quantumdot devices. This paper reports synthesis of PbS QDs by chemical route. The fabricated samples are characterized by UV-Vis, XRD, SEM, TEM, EDS, etc. Observed characteristics confirm nano formation. I-V characteristics of the sample are studied for investigating their applications as 'memristor'.

Semiconductor quantumdots (QDots) have historically been of interest to the scientific community since their creation for various applications ranging from solar energy to optical labeling. In this study, bioconjugated CdSe/ZnS core/shell QDots were synthesized and functionalized with 3-mercaptopropionic acid using both traditional ligand exchange as well as newly developed in situ functionalization techniques used to increase the quantum yield of the QDots. Their fluorescence and bonding to both gold as well as DNA origami were investigated for use in self assembled DNA constructs. It is believed that controlling the attachment and spacing of these nanoparticles on DNA origami could be used in a variety of optical labeling and sensing applications. Commercially available biotin and streptavidin functionalized quantumdots were also examined, and subject to the same experiments with gold nanoparticles as the MPA functionalized QDots.

A theoretical investigation is presented on the properties of charge transfer excitons at vertically coupled semiconductor quantumdots in the presence of electric and magnetic fields directed along the growth axis. Such excitons should have two interesting characteristics: an extremely long lifetime and a permanent dipole moment. We show that wave functions and the low-lying energies of charge transfer exciton can be found exactly for a special morphology of quantumdots that provides a parabolic confinement inside the layers. To take into account a difference between confinement potentials of an actual structure and of our exactly solvable model, we use the Galerkin method. The density of energy states is calculated for different InAs/GaAs quantum dots’ dimensions, the separation between layers, and the strength of the electric and magnetic fields. A possibility of a formation of a giant dipolar momentum under external electric field is predicted. PMID:23092373

Magneto-transport measurements on a triple quantumdot ring have recently shown anomalous quantum oscillations with dominant frequencies separated by a factor of three in magnetic flux [1]. Such oscillations, suggestive of a one-third periodicity in the flux quantum, are usually not observed in larger mesoscopic rings in which only larger periods are observed. We develop a microscopic transport model for the triple dot and show that the anomalous oscillations can dominate the transport behavior under certain conditions. Furthermore, we discuss the range of validity of our model by studying dephasing due to broadening and electric dipole interactions. [4pt] [1] L. Gaudreau et al., Phys. Rev. B 80, 075415 (2009)

We report on numerical simulations of InP surface lateral quantum-dot molecules on In0.48Ga0.52P buffer, using a model strictly derived by experimental results by extrapolation of the molecules shape from atomic force microscopy images. Our study has been inspired by the comparison of a photoluminescence spectrum of a high-density InP surface quantumdot sample with a numerical ensemble average given by a weighted sum of simulated single quantum-dot spectra. A lack of experimental optical response from the smaller dots of the sample is found to be due to strong inter-dot strain fields, which influence the optoelectronic properties of lateral quantum-dot molecules. Continuum electromechanical, k →.p → bandstructure, and optical calculations are presented for two different molecules, the first composed of two dots of nearly identical dimensions (homonuclear), the second of two dots with rather different sizes (heteronuclear). We show that in the homonuclear molecule the hydrostatic strain raises a potential barrier for the electrons in the connection zone between the dots, while conversely the holes do not experience any barrier, which considerably increases the coupling. Results for the heteronuclear molecule show instead that its dots do not appear as two separate and distinguishable structures, but as a single large dot, and no optical emission is observed in the range of higher energies where the smaller dot is supposed to emit. We believe that in samples of such a high density the smaller dots result as practically incorporated into bigger molecular structures, an effect strongly enforced by the inter-dot strain fields, and consequently it is not possible to experimentally obtain a separate optical emission from the smaller dots.

We report on numerical simulations of InP surface lateral quantum-dot molecules on In{sub 0.48}Ga{sub 0.52 }P buffer, using a model strictly derived by experimental results by extrapolation of the molecules shape from atomic force microscopy images. Our study has been inspired by the comparison of a photoluminescence spectrum of a high-density InP surface quantumdot sample with a numerical ensemble average given by a weighted sum of simulated single quantum-dot spectra. A lack of experimental optical response from the smaller dots of the sample is found to be due to strong inter-dot strain fields, which influence the optoelectronic properties of lateral quantum-dot molecules. Continuum electromechanical, k{sup →}·p{sup →} bandstructure, and optical calculations are presented for two different molecules, the first composed of two dots of nearly identical dimensions (homonuclear), the second of two dots with rather different sizes (heteronuclear). We show that in the homonuclear molecule the hydrostatic strain raises a potential barrier for the electrons in the connection zone between the dots, while conversely the holes do not experience any barrier, which considerably increases the coupling. Results for the heteronuclear molecule show instead that its dots do not appear as two separate and distinguishable structures, but as a single large dot, and no optical emission is observed in the range of higher energies where the smaller dot is supposed to emit. We believe that in samples of such a high density the smaller dots result as practically incorporated into bigger molecular structures, an effect strongly enforced by the inter-dot strain fields, and consequently it is not possible to experimentally obtain a separate optical emission from the smaller dots.

We demonstrate the development of a double quantumdot with an integrated charge sensor fabricated in undoped GaAs/AlGaAs heterostructures using a double top-gated design. Based on the evaluation of the integrated charge sensor, the double quantumdot can be tuned to a few-electron region. Additionally, the inter-dot coupling of the double quantumdot can be tuned to a large extent according to the voltage on the middle gate. The quantumdot is shown to be tunable from a single dot to a well-isolated double dot. To assess the stability of such design, the potential fluctuation induced by 1/f noise was measured. Based on the findings herein, the quantumdot design developed in the undoped GaAs/AlGaAs semiconductor shows potential for the future exploitation of nano-devices.

We present magnetotransport measurements in an open quantumdot realized on an InGaAs/InAlAs narrow quantum well. The measurements are performed on a 500 nm diameter circular cavity patterned by electron beam lithography and wet etching. The electronic density can be tuned by a Ti/Pt/Au electrostatic gate. The sample is characterized down to 300mK in a magnetic field up to 5T. We observe a superposition of slowly varying reproducible magnetoconductance fluctuations and a rich pattern of universal conductance fluctuations whose characteristic magnetic field scale is much shorter. We study the evolution of these two types of fluctuations as a function of the temperature (up to 230K) and the gate voltage. We notice the persistence of fluctuations up to unexpectedly high temperatures.

By a combination of Hartree-Fock simulations, exact diagonalization, and perturbative calculations, we investigate the ground-state properties of disorder-free circularquantumdots formed in a graphene monolayer. Taking the reference chemical potential at the Dirac point, we study N{<=}15 interacting particles, where the fine structure constant {alpha} parametrizes the Coulomb interaction. We explore three different models: (i) Sucher's positive projection (''no-pair'') approach, (ii) a more general Hamiltonian conserving both N and the number of additional electron-hole pairs, and (iii) the full quantum electrodynamics problem, where only N is conserved. We find that electron-hole pair production is important for {alpha} > or approx. 1. This corresponds to a reconstruction of the filled Dirac sea and is a finite-size version of the bulk excitonic instability. We also address the effects of an orbital magnetic field.

Fluorescence techniques have been extensively employed to develop non-invasive methodologies for tracking and understanding complex biological processes both in vitro and in vivo, which is of high importance in modern life science research. Among a variety of fluorescent probes, inorganic semiconductor quantumdots (QDs) have shown advantages in terms of better photostability, larger Stokes shift and more feasible surface functionalization. However, their intrinsic toxic heavy metal components and unstable fluorescence at low pH greatly impede the applications of QDs in in vivo studies. In this work, we developed novel fluorescent probes that can outperform currently available QD based probes in practice. Using conjugated oligomer with aggregation-induced emission characteristics as the fluorescent domain and biocompatible lipid-PEG derivatives as the encapsulation matrix, the obtained organic dots have shown higher brightness, better stability in biological medium and comparable size and photostability as compared to their counterparts of inorganic QDs. More importantly, unlike QD-based probes, the organic fluorescent dots do not blink, and also do not contain heavy metal ions that could be potentially toxic when applied for living biosubstrates. Upon surface functionalization with a cell-penetrating peptide, the organic dots greatly outperform inorganic quantumdots in both in vitro and in vivo long-term cell tracing studies, which will be beneficial to answer crucial questions in stem cell/immune cell therapies. Considering the customized fluorescent properties and surface functionalities of the organic dots, a series of biocompatible organic dots will be developed to serve as a promising platform for multifarious bioimaging tasks in future.

When electron spin and momentum couple in a solid, one generally obtains intriguing and unexpected phenomena. Metallic ferromagnetic nanotubes of cobalt with circular magnetization, which have been prepared by us and others, are a particularly interesting system. Here the spins of the conduction electrons are frustrated. They would like to align parallel to the magnetic field of the magnetization, but as the electrons move quickly around the tube the spins cannot follow the magnetization direction. In a previous short theoretical paper we solved the spin dynamics using a classical model. Here we generalize our work to a quantum mechanical model. The surprising result is that the spin of most conduction electrons is not parallel or anti-parallel to the circumferential magnetization but mostly parallel or anti-parallel to the axis of the nanotube. This result means that such a cobalt nanotube is a different ferromagnet from a cobalt film or bulk cobalt.

We investigate entanglement transfer from a system of two spin-entangled electron-hole pairs, each placed in a separate single mode cavity, to the photons emitted due to cavity leakage. Dipole selection rules and a splitting between the light hole and the heavy hole subbands are the crucial ingredients establishing a one-to-one correspondence between electron spins and circular photon polarizations. To account for the measurement of the photons as well as dephasing effects, we choose a stochastic Schrödinger equation and a conditional master equation approach, respectively. The influence of interactions with the environment as well as asymmetries in the coherent couplings on the photon entanglement is analysed for two concrete measurement schemes. The first one is designed to violate the Clauser-Horne-Shimony-Holt (CHSH) inequality, while the second one employs the visibility of interference fringes to prove the entanglement of the photons. Because of the spatial separation of the entangled electronic system over two quantumdots, a successful verification of entangled photons emitted by this system would imply the detection of nonlocal spin entanglement of massive particles in a solid state structure. PMID:20571188

We study the injection mechanism of a single electron from a static quantumdot into a moving quantumdot. The moving quantumdots are created with surface acoustic waves (SAWs) in a long depleted channel. We demonstrate that the injection process is characterized by an activation law with a threshold that depends on the SAW amplitude and on the dot-channel potential gradient. By sufficiently increasing the SAW modulation amplitude, we can reach a regime where the transfer has unity probability and is potentially adiabatic. This study points to the relevant regime to use moving dots in quantum information protocols.

We study the injection mechanism of a single electron from a static quantumdot into a moving quantumdot. The moving quantumdots are created with surface acoustic waves (SAWs) in a long depleted channel. We demonstrate that the injection process is characterized by an activation law with a threshold that depends on the SAW amplitude and on the dot-channel potential gradient. By sufficiently increasing the SAW modulation amplitude, we can reach a regime where the transfer has unity probability and is potentially adiabatic. This study points to the relevant regime to use moving dots in quantum information protocols. PMID:27087057

The patterning of colloidal quantumdots with nanometer resolution is essential for their application in photonics and plasmonics. Several patterning approaches, such as the use of polymer composites, molecular lock-and-key methods, inkjet printing and microcontact printing of quantumdots have been recently developed. Herein, we present a simple method of patterning colloidal quantumdots for photonic nanostructures such as straight lines, rings and dot patterns either on transparent or metallic substrates. Sub-10 nm width of the patterned line could be achieved with a well-defined sidewall profile. Using this method, we demonstrate a surface plasmon launcher from a quantumdot cluster in the visible spectrum. PMID:22895055

We show that fractional charges can be realized at the boundaries of a linear array of tunnel-coupled quantumdots in the presence of a periodically modulated onsite potential. While the charge fractionalization mechanism is similar to the one in polyacetylene, here the values of fractional charges can be tuned to arbitrary values by varying the phase of the onsite potential or the total number of dots in the array. We also find that the fractional boundary charges, unlike the in-gap bound states, are stable against static random disorder. We discuss the minimum array size where fractional boundary charges can be observed.

We experimentally investigate the charge (isospin) frustration induced by a geometrical symmetry in a triangular triple quantumdot. We observe the ground-state charge configurations of sixfold degeneracy, the manifestation of the frustration. The frustration results in omnidirectional charge transport, and it is accompanied by nearby nontrivial triple degenerate states in the charge stability diagram. The findings agree with a capacitive interaction model. We also observe unusual transport by the frustration, which might be related to elastic cotunneling and the interference of trajectories through the dot. This work demonstrates a unique way of studying geometrical frustration in a controllable way. PMID:25166188

Aromatic nitrogen doped graphene quantumdots were investigated by steady-state and time-resolved photoluminescence (PL) techniques. The PL lifetime was found to be dependent on the emission wavelength and coincident with the PL spectrum, which is different from most semiconductor quantumdots and fluorescent dyes. This result shows the synergy and competition between the quantum confinement effect and edge functional groups, which may have the potential to guide the synthesis and expand the applications of graphene quantumdots.

In this paper simultaneous effects of external electric and magnetic fields and quantum confinement on the radiation properties of spherical quantumdot embedded in a photonic crystal are investigated. Under the influence of photonic band-gap, effects of external static fields and dot dimension on the amplitude and spectrum of different radiation fields emitted by the quantumdot are studied. Our results show the considerable effects of external fields and quantum confinement on the spontaneous emission of the system.

Researchers at the National Renewable Energy Laboratory (NREL) have certified the first all-quantum-dot photovoltaic cell, which was based on lead sulfide and demonstrated reasonable quantumdot solar cell performance for an initial efficiency measurement along with good stability. The certified open-circuit voltage of the quantumdot cell is greater than that possible from bulk lead sulfide because of quantum confinement.

One potential, long-term approach to more efficient and lower cost future generation solar cells for solar electricity and solar fuels is to utilize the unique properties of quantumdots (QDs) to control the relaxation pathways of excited states to enhance multiple exciton generation (MEG). We have studied MEG in close-packed PbSe QD arrays where the QDs are electronically coupled in the films and thus exhibit good transport while still maintaining quantization and MEG. We have developed simple, all-inorganic solution-processable QD solar cells that produce large short-circuit photocurrents and power conversion efficiencies above 5% via nanocrystalline p-n junctions. These solar cells show QYs for photocurrent that exceed 100% in the photon energy regions where MEG is possible; the photocurrent MEG QYs as a function of photon energy match those determined via time-resolved spectroscopy Recent analyses of the major effect of MEG combined with solar concentration on the conversion efficiency of solar cells will also be discussed.

The growing presence of quantumdots (QD) in a variety of biological, medical, and electronics applications means an increased risk of human exposure in manufacturing, research, and consumer use. However, very few studies have investigated the susceptibility of skin to penetration of QD - the most common exposure route- and the results of those that exist are conflicting. This suggests that a technique allowing determination of skin barrier status and prediction of skin permeability to QD would be of crucial interest as recent findings have provided evidence of in vitro cytotoxicity and long-term in vivo retention in the body for most QD surface chemistries. Our research focuses on barrier status of the skin (intact and with ultraviolet radiation induced barrier defect) and its impact on QD skin penetration. These model studies are particularly relevant to the common application condition of NP containing sunscreen and SPF cosmetics to UV exposed skin. Herein we present our initial efforts to develop an in vivo model of nanoparticle skin penetration using the SKH-1 hairless mouse with transepidermal water loss (TEWL) to evaluate skin barrier status and determine its ability to predict QD penetration. Our results show that ultraviolet radiation increases both TEWL and skin penetration of QD. Additionally, we demonstrate cytotoxic potential of QD to skin cells using a metastatic melanoma cell line. Our research suggests future work in specific targeting of nanoparticles, to prevent or enhance penetration. This knowledge will be used to develop powerful therapeutic agents, decreased penetration cosmetic nanoparticles, and precise skin cancer imaging modalities.

An ordered lattice of lateral InAs quantumdot (QD) molecules is created by self-organized anisotropic strain engineering of an (In,Ga)As/GaAs superlattice (SL) template on GaAs(311)B by molecular-beam epitaxy, constituting a Turing pattern in solid state. The SL template and InAs QD growth conditions, such as the number of SL periods, growth temperatures, amount and composition of deposited (In,Ga)As, and insertion of Al-containing layers, are studied in detail for an optimized QD ordering within and among the InAs QD molecules on the SL template nodes, which is evaluated by atomic force microscopy. The average number of InAs QDs within the molecules is controlled by the thickness of the upper GaAs separation layer on the SL template and the (In,Ga)As growth temperature in the SL. The strain-correlated growth in SL template formation and QD ordering is directly confirmed by high-resolution x-ray diffraction. Ordered arrays of single InAs QDs on the SL template nodes are realized for elevated SL template and InAs QD growth temperatures together with the insertion of a second InAs QD layer. The InAs QD molecules exhibit strong photoluminescence (PL) emission up to room temperature. Temperature-dependent PL measurements exhibit an unusual behavior of the full width at half maximum, indicating carrier redistribution solely within the QD molecules.

We discuss the optical response of a quantum molecule under the action of two lasers fields. Using a realistic model and parameters, we map the physical conditions to find three different phenomena reported in the literature: the tunneling induced transparency, the formation of Autler-Townes doublets, and the creation of a Mollow-like triplet. We found that the electron tunneling between quantumdots is responsible for the different optical regime. Our results not only explain the experimental results in the literature but also give insights for future experiments and applications in optics using quantumdots molecules.

This viewgraph presentation reviews the modeling of Quantum Well Infrared Detectors (QWIP) and QuantumDot Infrared Detectors (QDIP) in the development of Focal Plane Arrays (FPA). The QWIP Detector being developed is a dual band detector. It is capable of running on two bands Long-Wave Infrared (LWIR) and Medium Wavelength Infrared (MWIR). The same large-format dual-band FPA technology can be applied to QuantumDot Infrared Photodetector (QDIP) with no modification, once QDIP exceeds QWIP in single device performance. Details of the devices are reviewed.

In this work, undoped and Ag-doped ZnS quantumdots were synthesized using various chemical methods. The products were characterized using X-ray diffraction (XRD), UV-visible spectroscopy and Photoluminescence spectroscopy. Our results revealed that the size of the as-prepared samples range from 1–6 nm in diameter and have a cubic zinc-blende structure. Also, we observed the emission of different wavelength of light from different sized quantumdots of the same material due to quantum confinement effect. The results will be presented in detail and ZnS can be a potential candidate for optical device development and applications.

Ultrafast density-dependent optical spectroscopic measurements on a quantumdots-in-a-well heterostructure reveal several distinctive phenomena, most notably a strong coupling between the quantum well population and light absorption at the quantumdot excited state.

In-depth understanding of the nature of cell physiology and ability to diagnose and control the progression of pathological processes heavily rely on untangling the complexity of intracellular molecular mechanisms and pathways. Therefore, comprehensive molecular profiling of individual cells within the context of their natural tissue or cell culture microenvironment is essential. In principle, this goal can be achieved by tagging each molecular target with a unique reporter probe and detecting its localization with high sensitivity at sub-cellular resolution, primarily via microscopy-based imaging. Yet, neither widely used conventional methods nor more advanced nanoparticle-based techniques have been able to address this task up to date. High multiplexing potential of fluorescent probes is heavily restrained by the inability to uniquely match probes with corresponding molecular targets. This issue is especially relevant for quantumdot probes---while simultaneous spectral imaging of up to 10 different probes is possible, only few can be used concurrently for staining with existing methods. To fully utilize multiplexing potential of quantumdots, it is necessary to design a new staining platform featuring unique assignment of each target to a corresponding quantumdot probe. This dissertation presents two complementary versatile approaches towards achieving comprehensive single-cell molecular profiling and describes engineering of quantumdot probes specifically tailored for each staining method. Analysis of expanded molecular profiles is achieved through augmenting parallel multiplexing capacity with performing several staining cycles on the same specimen in sequential manner. In contrast to other methods utilizing quantumdots or other nanoparticles, which often involve sophisticated probe synthesis, the platform technology presented here takes advantage of simple covalent bioconjugation and non-covalent self-assembly mechanisms for straightforward probe

Optical antennas based on noble metal nanoparticles can increase the photoluminescence of quantumdots, but the exact strength of this enhancement depends on the brightness (i.e., the intrinsic quantum yield ηi ) of the emitters. Here we perform temperature-dependent measurements on a system of PbS colloidal quantumdots coupled with Au ring arrays that bring quantitative insight into this phenomenon. We show that although the boost in photoluminescence is lower at cryogenic temperatures where the nanocrystals become very bright emitters, the spectral signature of this enhancement is remarkably independent of ηi. These observations remain true even at wavelengths where the losses by absorption in the metal nanoparticles considerably increase due to the excitation of localized plasmon resonances, in contradiction with standard theory that treats the emitters as a collection of two-level systems. We propose a mechanism in which the quantumdots are modeled as multilevel and inhomogeneously broadened emitters to account for these findings.

Here we investigate the influence of a constant magnetic field on the energy levels and optical properties of excitons in a double quantum disk (quantum molecule). Taking into account the cylindrical symmetry of the double disk we calculate the wave functions and excitonic energies when the external constant magnetic field is applied along the symmetry axis. Having the eigenfunctions and eigenvalues and using the long-wave approximation we can compute all the optical functions. The double quantumdot is considered as one system rather than two interacting dots separated by narrow barrier. The screened Coulomb interaction between an electron and a hole is assumed. Since in the given structure the separation of the relative- and center-of-mass motion of the electron and the hole is not possible, we use an approach where the six-dimensional eigenvalue problem is transformed into the equivalent eigenvalue problem given by the system of the coupled two-dimensional second order differential equations. The so obtained differential equations are solved numerically. As an example, we give detailed results for a InP/InGaP double quantumdot. Satisfactory agreement with the available experimental data is obtained.

Controlling the size of semiconductor nanostructures allows manipulation of the optical and electrical properties of band carriers. We show that laterally-confined monolayer MoS2 quantumdots can be created through top-down nanopatterning of an atomically-thin two-dimensional semiconductor. Semiconductor-compatible nanofabrication processing allows for these low-dimensional materials to be integrated into complex systems that harness their controllable optical properties. Size-dependent exciton energy shifts and linewidths are observed, demonstrating the influence of quantum confinement. The patterned dots exhibit the same valley polarization characteristics as in a continuous MoS2 sheet, suggesting that monolayer semiconductor quantumdots could have potential for advancing quantum information applications. This work is supported by ISEN, the DOE-BES (DE-SC0012130), the NSF MRSEC program (DMR-1121262), and the Center for Nanoscale Materials, DOE-BES (DE-AC02-06CH11357). N.P.S. is an Alfred P. Sloan Research Fellow.

We present a detailed theoretical study of linear and third order nonlinear refractive index changes in a optically driven disk-like GaN quantumdot. In our numerical calculations, we consider the three level system containing biexciton, exciton, and ground states and use the compact density matrix formalism and iterative method to obtain refractive index changes. Variational method through effective mass approximation are employed to calculate the ground state energy of biexciton and exciton states. The evolution of refractive index changes around one, two and three photon resonance is investigated and discussed for different quantumdot sizes and light intensities. Size-dependent three-photon nonlinear refractive index change versus incident photon energy compared to that of two-photon is obtained and analyzed. As main result, we found that around resonance frequency at exciton-biexciton transition the quantum confinement has great influence on the linear change in refractive index so that for very large quantumdots, it decreases. Moreover, it was found that third order refractive index changes for three photon process is strongly dependent on QD size and light intensity. Our study reveals that considering our simple model leads to results which are in good agreement with other rare numerical results. Comparison with experimental results has been done.

We successfully developed a method utilizing colloidal CdSe nanocrystalline quantumdot for anti-counterfeiting label on a piece of glossy paper. We deposited numbers and lines patterns of toluene soluble CdSe quantumdot using rubber stamper on a glossy paper. The width of line pattern was about 1-2 mm with 1-2 mm separation between lines. It required less than one minute for deposited CdSe quantumdot on glossy paper to dry and become invisible by naked eyes. However, patterned quantumdot become visible using long-pass filter glasses upon excitation of UV lamp or blue laser. We characterized photoluminescence of line patterns of quantumdot, and we found that emission boundaries of line patterns were clearly observed. The error of line size and shape were mainly due to defect of the original stamper. The emission peak wavelength of CdSe quantumdot was 629 nm. The emission spectrum of deposited quantumdot has full width at half maximum (FWHM) of 30-40 nm. The spectra similarity between deposited quantumdot and the original quantumdot in solution proved that our stamping method can be simply applied on glossy paper without changing basic optical property of the quantumdot. Further development of this technique is potential for anti-counterfeiting label on very important documents or objects.

Short, histidine-containing peptides can be conjugated to lysine-containing protein scaffolds to controllably attach quantumdots (QDs) to the scaffold, allowing for generic attachment of quantumdots to any protein without the use of specially engineered domains. This technique was used to bind quantumdots from aqueous solution to both chicken IgG and cowpea mosaic virus (CPMV), a 30 nm viral particle. These quantumdot protein assemblies were studied in detail. The IgG QD complexes were shown to retain binding specificity to their antigen after modification. The CPMV QD complexes have a local concentration of quantumdots greater than 3000 nmol ml-1, and show a 15% increase in fluorescence quantum yield over free quantumdots in solution.

Depending on the minimum size of their micro/nano structure, thin films can exhibit very different behaviors and optical properties. From optical waveguides down to artificial anisotropy, through diffractive optics and photonic crystals, the application changes when decreasing the minimum feature size. Rigorous electromagnetic theory can be used to model most of the components but when the size is of a few nanometers, quantum theory has also to be used. These materials including quantum structures are of particular interest for other applications, in particular for solar cells, because of their luminescent and electronic properties. We show that the properties of electrons in multiple quantum wells can be easily modeled with a formalism similar to that used for multilayer waveguides. The effects of different parameters, in particular coupling between wells and well thickness dispersion, on possible discrete energy levels or energy band of electrons and on electron wave functions is given. When such quantum confinement appears the spectral absorption and the extinction coefficient dispersion with wavelength is modified. The dispersion of the real part of the refractive index can then be deduced from the Kramers- Krönig relations. Associated with homogenization theory this approach gives a new model of refractive index for thin films including quantumdots. Absorption spectra of samples composed of ZnO quantumdots in PMMA layers are in preparation are given.

Self-assembled InN quantumdots (QDs) were grown on Si(111) substrate using plasma assisted molecular beam epitaxy (PA-MBE). Single-crystalline wurtzite structure of InN QDs was confirmed by X-ray diffraction. The dot densities were varied by varying the indium flux. Variation of dot density was confirmed by FESEM images. Interdigitated electrodes were fabricated using standard lithog- raphy steps to form metal-semiconductor-metal (MSM) photodetector devices. The devices show strong infrared response. It was found that the samples with higher density of InN QDs showed lower dark current and higher photo current. An explanation was provided for the observations and the experimental results were validated using Silvaco Atlas device simulator. PMID:27398511

We discuss a ``gentle`` electrochemical technique for fabricating quasi-periodic quantumdot arrays. The technique exploits a self-organizing phenomenon to produce quasi-periodic arrangement of dots and provides excellent control over dot size and interdot spacing. Unlike conventional nanolithography, it does not cause radiation damage to the structures during exposure to pattern delineating beams (e-beam, ion-beam or x-ray). Moreover, it does not require harsh processing steps like reactive ion etching, offers a minimum feature size of {approximately}40 {angstrom}, allows the fabrication of structures on nonplanar surfaces (e.g. spherical or cylindrical substrates), is amenable to mass production (millions of wafers can be processed simultaneously) and is potentially orders of magnitude cheaper than conventional nanofabrication. In this paper, we describe our initial results and show the promise of this technique for low-cost and high-yield nanosynthesis.

We study theoretically Coulomb drag in capacitively coupled quantumdots (CQDs)-a bias-driven dot coupled to an unbiased dot where transport is due to Coulomb mediated energy transfer drag. To this end, we introduce a master-equation approach that accounts for higher-order tunneling (cotunneling) processes as well as energy-dependent lead couplings, and identify a mesoscopic Coulomb drag mechanism driven by nonlocal multielectron cotunneling processes. Our theory establishes the conditions for a nonzero drag as well as the direction of the drag current in terms of microscopic system parameters. Interestingly, the direction of the drag current is not determined by the drive current, but by an interplay between the energy-dependent lead couplings. Studying the drag mechanism in a graphene-based CQD heterostructure, we show that the predictions of our theory are consistent with recent experiments on Coulomb drag in CQD systems. PMID:27232031

We study theoretically Coulomb drag in capacitively coupled quantumdots (CQDs)—a bias-driven dot coupled to an unbiased dot where transport is due to Coulomb mediated energy transfer drag. To this end, we introduce a master-equation approach that accounts for higher-order tunneling (cotunneling) processes as well as energy-dependent lead couplings, and identify a mesoscopic Coulomb drag mechanism driven by nonlocal multielectron cotunneling processes. Our theory establishes the conditions for a nonzero drag as well as the direction of the drag current in terms of microscopic system parameters. Interestingly, the direction of the drag current is not determined by the drive current, but by an interplay between the energy-dependent lead couplings. Studying the drag mechanism in a graphene-based CQD heterostructure, we show that the predictions of our theory are consistent with recent experiments on Coulomb drag in CQD systems.

A simple device of three laterally coupled quantumdots, the central one contacted by metal leads, provides a realization of the ferromagnetic Kondo model, which is characterized by interesting properties like a nonanalytic inverted zero-bias anomaly and an extreme sensitivity to a magnetic field. Tuning the gate voltages of the lateral dots allows us to study the transition from a ferromagnetic to antiferromagnetic Kondo effect, a simple case of a Berezinskii-Kosterlitz-Thouless transition. We model the device by three coupled Anderson impurities that we study by numerical renormalization group. We calculate the single-particle spectral function of the central dot, which at zero frequency is proportional to the zero-bias conductance, across the transition, both in the absence and in the presence of a magnetic field.

A simple device of three laterally coupled quantumdots, the central one contacted by metal leads, provides a realization of the ferromagnetic Kondo model, which is characterized by interesting properties like a nonanalytic inverted zero-bias anomaly and an extreme sensitivity to a magnetic field. Tuning the gate voltages of the lateral dots allows us to study the transition from a ferromagnetic to antiferromagnetic Kondo effect, a simple case of a Berezinskii-Kosterlitz-Thouless transition. We model the device by three coupled Anderson impurities that we study by numerical renormalization group. We calculate the single-particle spectral function of the central dot, which at zero frequency is proportional to the zero-bias conductance, across the transition, both in the absence and in the presence of a magnetic field. PMID:23931401

We studied the emission of quantumdots in the presence of plasmon-metal oxide substrates, which consist of arrays of metallic nanorods embedded in amorphous silicon coated with a nanometer-thin layer of aluminum oxide on the top. We showed that the combined effects of plasmons and the silicon-aluminum oxide interface can lead to significant enhancement of the quantum efficiency of quantumdots. Our results show that such an interface can significantly enhance plasmonic effects of the nanorods via quantumdot-induced exciton-plasmon coupling, leading to partial polarization of the quantumdots' emission.

A quantumdot memory based on a GaAs/AlGaAs quantum wire with site-controlled InAs quantumdots was realized by means of molecular beam epitaxy and etching techniques. By sampling of different gate voltage sweeps for the determination of charging and discharging thresholds, it was found that discharging takes place at short time scales of μs, whereas several seconds of waiting times within a distinct negative gate voltage range were needed to charge the quantumdots. Such quantumdot structures have thus the potential to implement logic functions comprising charge and time dependent ingredients such as counting of signals or learning rules.

We have fabricated a few-electron quantumdot that can be tuned down to zero electrons while maintaining strong coupling to the leads. Using a nearby quantum point contact as a charge sensor, we can determine the absolute number of electrons in the quantumdot. We find several sharp peaks in the differential conductance, occurring at both zero and finite source-drain bias, for the one- and two-electron quantumdot. We attribute the peaks at finite bias to a Kondo effect through excited states of the quantumdot and investigate the magnetic field dependence of these Kondo resonances. PMID:16712183

Longitudinal photoconductivity spectra of Si/Ge multilayer structures with Ge quantumdots grown pseudomorphically to the Si matrix are studied. Lines of optical transitions between hole levels of quantumdots and Si electronic states are observed. This allowed us to construct a detailed energy-level diagram of electron-hole levels of the structure. It is shown that hole levels of pseudomorphic Ge quantumdots are well described by the simplest 'quantum box' model using actual sizes of Ge islands. The possibility of controlling the position of the long-wavelength photosensitivity edge by varying the growth parameters of Si/Ge structures with Ge quantumdots is determined.

The lateral photoconductivity spectra of Si/Ge multilayer structures with Ge quantumdots of various sizes are investigated. We observed optical transition lines between the hole levels of quantumdots and electronic states of Si. This enabled us to construct a detailed energy level diagram of the electron-hole spectrum of the Si/Ge structures. It is shown that the hole levels of Ge quantumdots are successfully described by the 'quantum box' model using the actual sizes of Ge islands. It I found that the position of the longwavelength photosensitivity boundary of Si/Ge structures with Ge quantumdots can be controlled by changing the growth parameters.

Intermediate band solar cell (IBSC) has become a promising technology in increasing solar cell efficiency. In this work we compare absorption coefficient profile between InAs quantumdots with GaAs bulk. We calculate the efficiency of GaAs bulk and GaAs doped with 2, 5, and 10 nm InAs quantumdot. Effective distances in quantumdot arrangement based on electron tunneling consideration were also calculated. We presented a simple calculation method with low computing power demand. Results showed that arrangement of quantumdot InAs in GaAs can increase solar cell efficiency from 23.9 % initially up to 60.4%. The effective distance between two quantumdots was found 2 nm in order to give adequate distance to prevent electron tunneling and wave functions overlap.